The saying “two can live as cheaply as one” has been cited in print since at least the 1890s. “Two can eat as cheaply as one” has been cited in print since at least 1927. Restaurant advertisements in the 1930s offered specials for two people (such as “2 for 65c”), stating that “two can eat as cheaply as one.”
 
 
16 February 1895, Biloxi (MS) Herald, pg. 3, col. 6:
UNSHACKLED—“I deduct from the census report that two can live as cheaply as one.” Shackled—“When you add to the census report you will see the difference.”—Philadelphia Record.
 
21 February 1927, San Diego (CA) Union, “Roundabouts” edited by A. Roundabouter, pg. 4, col. 6:
Dear Roundabouts: The question: “Can two live as cheaply as one?” has frequently found a place in your column. In our case the answer is YES! The first Monday after we were married, my husband asked me how much money I wanted to run the house for a week. I asked him how much it had cost him to live at restaurants for that period of time. He mentioned a figure that I considered rather large, so I said “give me that amount, and we’ll see how far that will carry us.”
 
Hubby reads the column, so I am not going to say how much I have left over. But he says he is living better than he did at restaurants. He gained 22 pounds in less than seven months, which is pretty conclusive proof that he is not undernourished.
 
Yes, Mr. Roundabouter, two can eat as cheaply as one!
A. C. E.
 
14 July 1928, Cleveland (OH) Plain Dealer, “Two Can Live as Cheap as One? That Theory Is All Wet” by Dorothy Dix, pg. 12, col. 8:
Forget the two-can-live-as-cheaply-as-one argument, because they can’t.
   
Google News Archive
13 January 1934, Woodville (MS) Republican, “Plenty Wise,” pg. 4, col. 4
Utopian — The dream that two can eat as cheaply as one.
 
Google News Archive
2 September 1938, Pittsburgh (PA) Post-Gazette, pg. 2, col. 2 ad:
Were Washington living today, he would have decreed…“Boys, after the parade we’ll stay I town, because two can eat as cheaply as one at Reymer’s!”
YES…and Here’s How It’s Done…
DINNERS
2 for 65c
Reymer’s
 
Google News Archive
10 October 1940, Pittsburgh (PA) Press, pg. 6, col. 8 ad:
Two can eat as cheaply as one.
Friday’s the day to eat lunch at McCanns. Two persons can eat for 56 cents if they come together.
 
Google Books
This Is the Samoyed
By Joan McDonald Brearley
Neptune City, NJ: T.F.H. Publications
1975
Pg. 281:
We are certain that the expression “two can eat as cheaply as one” originated after some curious scientist made a detailed study of the life cycle of the flea.
   
Google Books
I’ll Take Manhattan
By Judith Krantz
New York, NY: Crown Publishers
1986
Pg. 24:
“There’s plenty of room in your apartment, two can eat as cheaply as one, girls are willing to go Dutch, and I walk to work anyway.”