Two cities that are close neighbors are sometimes called “twin cities.” ““Our line commences at the twin cities of Pittsburgh and Allegheny, the metropolis of Western Pennsylvania” was cited in the American Railroad Journal in 1849. Davenport (IA) and Rock Island (IL) were called “twin cities” in 1855.
 
Minneapolis (MN) was named in 1852. “St. Anthony and Minneapolis, the twin cities of the Falls” was cited in 1857. “The ‘twin cities’ of St. Anthony and Minneapolis” was cited in 1868. St. Anthony and Minneapolis were still called “twin cities” in 1872; that year, St. Anthony merged with Minneapolis.
 
Minneapolis and St. Paul were called the “twin cities” by at least 1876. The Minnesota Twins baseball team began in 1961, with the nickname inspired from the “Twin Cities” name.
 
     
Wikipedia: Minneapolis-Saint Paul
Minneapolis–Saint Paul is a major metropolitan area built around the Mississippi, Minnesota and St. Croix rivers. The area is commonly known as the Twin Cities for its two largest cities, Minneapolis, the city with the largest population in Minnesota, and Saint Paul, the state capital. It is a classic example of twin cities in the sense of geographical proximity. Together the two cities anchor the second-largest economic center in the Midwest, behind Chicago.
 
Wikipedia: Minneapolis
Minneapolis (/ˌmɪniˈæpəlɪs/) is the county seat of Hennepin County, and larger of the Twin Cities, the 14th-largest metropolitan area in the United States, containing approximately 3.8 million residents. As of 2016, Minneapolis is the largest city in the state of Minnesota and 46th-largest in the United States with 407,207 residents. Minneapolis and Saint Paul anchor the second-largest economic center in the Midwest, behind Chicago.
 
Minneapolis lies on both banks of the Mississippi River, just north of the river’s confluence with the Minnesota River, and adjoins Saint Paul, the state’s capital.
 
Wikipedia: Saint Paul, Minnesota
Saint Paul (/ˌseɪnt ˈpɔːl/; abbreviated St. Paul) is the capital and second-most populous city of the U.S. state of Minnesota. As of 2014, the city’s estimated population was 297,640. Saint Paul is the county seat of Ramsey County, the smallest and most densely populated county in Minnesota. The city lies mostly on the east bank of the Mississippi River in the area surrounding its point of confluence with the Minnesota River, and adjoins Minneapolis, the state’s largest city. Known as the “Twin Cities”, the two form the core of Minneapolis–Saint Paul, the 16th-largest metropolitan area in the United States, with about 3.5 million residents.
 
Wikipedia: St. Anthony, Minnesota
St. Anthony was also the name of the older twin city of Minneapolis, located across from downtown Minneapolis on the east bank of the Mississippi River. Minneapolis and St. Anthony merged in 1872
 
(Oxford English Dictionary)
twin city  n.  (a) N. Amer. either of two cities that are very close neighbours; spec. in pl. (U.S.) St. Paul and Minneapolis, (Canad.) Fort William and Port Arthur;  (b) occas. used of a city in the sense of twin town below.
1856   Rock Island (Illinois) Argus 23 Apr.  The church bells of the twin-cities [sc. Rock Island, Ill. and Davenport, Iowa] rang out their joyous notes in honor of the achievement [sc. bridging the Mississippi].
1883   Harper’s Mag. June 73/2   The twin cities [sc. St. Paul and Minneapolis, Minn.]..emulate each other in metropolitan airs.
 
OCLC WorldCat record
The secrets of the twin cities : or, the great metropolis unmasked : a startling story of city scenes in Boston and New York
Author: Charles E Averill
Publisher: Boston : George H. Williams, 1849.
Edition/Format:   Book   Microform : Microfilm
 
21 July 1849, American Railroad Journal, “Ohio and Pennsylvania Railroad,” pg. 459, col. 2:
“Our line commences at the twin cities of Pittsburgh and Allegheny, the metropolis of Western Pennsylvania, ...
 
16 January 1851, North American and United States Gazette, “Ohio and Pennsylvania Railroad Co,” pg. 1, col. 6:
The population of the twin cities of Pittsburg and Allegheny, ...
 
20 July 1855, Bangor (ME) Daily Whig and Courier, “Letter from Iowa” by H. B. M., pg. 2, col. 3:
There is a high bluff just back of the city, which overlooks the “twin cities”—Davenport and Rock Island.
 
OCLC WorldCat record
The Twin cities directory : including Davenport, Iowa, East Davenport, Iowa, Rock Island, Ill., Moline, Ill., and Camden, Ill.
Author: Fleming & Torrey.
Publisher: Davenport, Iowa : A.P. Luse & Co., 1856.
Edition/Format:   Print book : English
 
22 August 1857, Boston (MA) Daily Traveler, “From Minnesota,” pg. 3, col. 6:
First, of course, come St. Anthony and Minneapolis, the twin cities of the Falls, together about equal in population to St. Paul, namely 12,000.
 
5 December 1857, Weekly Minnesotian

Where would be the rising greatness of St. Anthony and Minneapolis if their dormant energies had not been infused with life by the money of those comprehensive minds who are making those twin cities the manufacturing centre of the North-West?
 
6 August 1858, Milwaukee (WI) Daily Sentinel, “The Excursion to St. Paul,” pg. 2, col. 1:
... in short nothing is wanting to the prosperous future of these twin cities, but the growth and development of Minnesota, and that, of course, will come in time.
 
Chronicling America
9 December 1858, St. Cloud (MN) Democrat, “The ‘Winslow House,’” pg. 3, col. 1:
The twin cities of St. Anthony and Minneapolis lay at our feet, ...
 
OCLC WorldCat record
Twin cities directory and business mirror, for the year 1860. Including the cities of Davenport, Rock Island and Moline, vol. 1.
Author: E. Coy & Co.
Publisher: Davenport, Luse, 1859.
Edition/Format:   Print book : English
 
23 August 1859, Providence (RI) Daily Evening Press, “Vacation Waifs,” pg. 2, col. 3:
The suspension bridge that connects S. Anthony with Minneapolis, twin cities, though of unequal size, is a graceful structure that you pay a dime to cross and re-cross!
 
OCLC WorldCat record
Brigham’s twin cities directory and business advertiser for 1861 & 1862 : containing a complete list of all the residents in the Twin Cities with many local and general statistics : including the cities of Davenport, Rock Island & Moline
Author: A Delancey Brigham
Publisher: Davenport, Iowa : Luse, Lane & Co., 1861.
Series: (United States city directories, 1861- ; no. 59).
Edition/Format:   Book   Microform : Microfilm : English
 
18 July 1865, Bangor (ME) Daily Whig and Courier, pg. 2, col. 2:
... Norfolk or Portsmouth; and further, that there are no apprehensions of this disease visting the “twin cities” again.
 
25 June 1867, Milwaukee (WI) Daily Sentinel, “The Editorial Excursion,” pg. 2, col. 4:
Minneapolis and St. Anthony, sitting on opposite banks of the Mississippi, have been called the “twin cities of the Upper Mississippi.”
 
7 March 1868, Chicago (IL) Tribune, “Minnesota: A Winter in St. Paul,” pg. 2, col. 7:
By special invitation of the Committee of invitation, on the part of the “twin cities” of St. Anthony and Minneapolis, I accompanied the Legislature to those cities to a public dinner at the Nicolet House, in Minneapolis, which, by the way, is one of the best hotels this side of Chicago.
 
Chronicling America
4 January 1872, St. Cloud (MN) Journal, pg. 3, col. 1:
This, it (Minneapolis Tribune—ed.) claims, shows a greater sum expended at the “twin cities” (Minneapolis and St. Anthony—ed.) for improvements than at any other place west of Chicago.
 
Chronicling America
5 January 1876, New-York (NY) Daily Tribune, “A High-Toned ‘Banquet,’” pg. 4, col. 3:
A Union of Hearts and Clasping of Hands between St. Paul and Minneapolis. (...) The Representative Men of the Twin Cities exchange greetings; ...
 
Chronicling America
4 May 1876, St. Cloud (MN) Journal, pg. 2, col. 2:
Let the brotherly love continue. The St. Paul and Minneapolis newspaper consolidation business has created such as beatific state of feeling in the two communities that the colored debating societies of the twin cities now hold consolidated meetings to discuss “de momentuous questions ob de day.”
 
11 September 1877. The Courier-Journal (Louisville, KY), “Minnesota; Quarrelsome Twins,” pg. 3, col. 5:
ST. PAUL, MINN. Sept. 5, 1877.—They call St. Paul and Minneapolis the “twin cities,” which is a very pretty conceit, barring he fact that these twins have been fighting and quarreling from the womb period to their present adolescence.
   
3 September 1881, Duluth (MN) Tribune, “What Will Be Duluth’s Share!.” pg. 2, col. 3:
But the speech he made, so terse and straight-forward, so free from exaggeration, has its meaning for Duluth, as well as for the cities of St. Paul and Minneapolis. Every word he says applies, or ought to apply, to the city at the head of the lakes, with as much force as to the twin cities further south.
 
24 September 1881, Boston (MA) Journal (Supplement(, “The Great Air Line,” pg. 5, col. 6:
The twin cities of Minneapolis and St. Paul were below them, connected by bonds of iron and the winding Mississippi River, while all about were rich and luxuriant farms.
 
OCLC WorldCat record
Twin City map.
Author: Davison, C. Wright,; Brown, Treacy & Co. (Saint Paul, Minn.)
Publisher: [Minnesota?] : C. Wright Davison, St. Paul, Minn. : Brown, Tracey & Co. 1886.
Edition/Format:   Map : English
 
OCLC WorldCat record
Our twin cities of the nineteenth century (Norfolk and Portsmouth) : their past, present, and future
Author: Robert W Lamb
Publisher: Norfolk, Va. : Barcroft, 1887-8.
Edition/Format:   Print book : English
 
OCLC WorldCat record
Illustrated atlas of the twin cities, Sault Ste. Marie, Michigan and Ontario : containing general and detail maps, showing streets and sewers and water mains, subdivisions with lots and their dimensions, private claims, etc, etc. : also maps of the hemispheres, world, United States and Michigan, with early and present history of the twin cities and surrounding country.
Author: William C Sauer
Publisher: Detroit, Mich. : Wm. C. Sauer, 1888.
Edition/Format:   Map : Atlas : Biography   Book : English
 
OCLC WorldCat record
The past and present of the Twin Cities : being an historical, financial, and commercial compend, or brief review of their origin, primitive days, rapid growth, immense resources, industries, and the elements constituting them one dual city ...
Author: Frank Chapman Bliss; F.C. Bliss & Co.
Publisher: St. Paul : [F.C. Bliss & Co.], [1888]
Edition/Format:   Print book : English
 
OCLC WorldCat record
25 miles around the Twin Cities
Author: George W Cooley; C Wright Davison; Smith & Harrison (Firm : Minneapolis, Minn.); R.L. Polk & Co.
Publisher: [Minneapolis] : C. Wright Davison, 1889, 1881
Edition/Format:   Map : English
 
OCLC WorldCat record
A tale of the Twin cities : lights and shadows of the street car strike in Minneapolis and St. Paul, Minnesota, beginning April 11, 1889
Author: Eva Gay
Publisher: Minneapolis : [T.A. Clark?], 1889.
Series: (Wright American fiction ; v. 3 (1876-1900), reel G-7, no. 2138).
Edition/Format:   Book   Microform : Fiction : Microfilm : English : 1st ed
 
OCLC WorldCat record
The twin cities of the Northwest.
Author: Charles King
Publisher: [N.Y.], [1890]
Edition/Format:   Print book : English
Subjects
Saint Paul (Minn.)—Description and travel.
Minneapolis (Minn.)—Description and travel.
Travel.