Gunnar Myrdal (1898-1987) and Friedrich von Hayek (1899-1992) shared the 1974 Nobel Prize in Economics, even though the former supported left-wing socialism and the latter supported right-wing free market capitalism. A joke was cited in November 1995:
 
“Economics is the only field in which two people can get a Nobel Prize for saying exactly the opposite thing.”
 
The idea behind the joke had been around since at least 1974, but the joke is a decade older. According to Roberto Alazar (see email, below), he first told the joke in the middle or late 1980s.
 
   
Wikipedia: Nobel Memorial Prize in Economic Sciences
The Nobel Memorial Prize in Economic Sciences (officially Swedish: Sveriges riksbanks pris i ekonomisk vetenskap till Alfred Nobels minne, or the Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel), commonly referred to as the Nobel Prize in Economics, is an award for outstanding contributions to the field of economics, and generally regarded as the most prestigious award for that field.
 
The prize was established in 1968 by a donation from Sweden’s central bank, the Sveriges Riksbank, on the bank’s 300th anniversary. Although it is not one of the prizes that Alfred Nobel established in his will in 1895, it is referred to along with the other Nobel Prizes by the Nobel Foundation. Winners are announced with the other Nobel Prize winners, and receive the award at the same ceremony.
 
Laureates in the Memorial Prize in Economics are selected by the Royal Swedish Academy of Sciences.
 
14 November 1995, The Gaston Gazette (Gastonia, NC), “Even economists have ticklish funny bones,” pg. 4D, col. 3:
“Economics is the only field in which two people can get a Nobel Prize for saying exactly the opposite thing.”
 
Google Books
Charlatans Or Saviours?:
Economists and the British Economy from Marshall to Meade
By Roger Middleton
Edward Elgar
1998
Pg. 1:
Economics is the only field in which two people can get a Nobel Prize for saying exactly the opposite thing.
   
Google Books
The Fortune Sellers:
The Big Business of Buying and Selling Predictions
By William A. Sherden
New York, NY: John Wiley & Sons, Inc.
1998
Pg. 61:
The beliefs are so varied and conflicting that economics is jokingly said to be the only field in which two people can get a Nobel Prize for saying exactly the opposite thing. And there is truth to the joke: James Tobin, who won the Nobel Prize in 1981, believes that international capital flows are too volatile and should be restricted, while Robert Lucas, who won the Nobel Prize in 1995, believes that markets should be free from government intervention. Derived from these differences is the First Law of Economics: For every economist, there is an equal and opposite economist.
 
Twitter
CPI
‏@CompPolicyInt
Thought for the day: Economics is the only field in which two people can get a Nobel Prize for saying exactly the opposite thing
2:11 PM - 7 Jul 2009
 
Twitter
Swift Economics
‏@SwiftEconomics
Economics is the only field in which 2 people can share a Nobel Prize for saying opposing things (Myrdahl & Hayek shared one in ‘74)
2:30 PM - 3 Jun 2011
 
Twitter
Luis
‏@IntellectualExp
“Economics is the only field in which two people can share a Nobel Prize for saying opposing things.” Roberto Alazar
4:25 PM - 12 Nov 2014
 
Twitter
Atheist+Zionist
‏@atheistrew
@edumak8 @forthleft @johndory49 Myrdal and Hayek shared a Nobel prize. Economics is the only science where this could happen.
5:33 AM - 16 Dec 2014
 
Twitter
Jay Mooreland
‏@EmotionalInvest
@ritholtz “#Economics is the only field in which two people can get a #Nobel Prize for saying exactly the opposite thing.” -Anonymous
8:54 PM - 27 May 2015
 
Forbes.com
OCT 17, 2015 @ 10:01 AM
How To Invest In The Stock Market Like A Brainiac Economist
By Ky Trang Ho
Corporations and government agencies hire brainiac economists with doctorate degrees to guide them through a morass of hundreds — if not thousands of numbers — to help develop policies and make investment decisions. Despite unwavering respect for their insights, economists are notorious for looking at the same data and coming to polar opposite conclusions. As the old joke goes: “Economics is the only field in which two people can win a Nobel Prize for saying the opposite thing.”
 
(Email from Roberto Alazar in June 2016)
As to the date, it would have been in the late-middle 1980’s.
 
Circumstance: It was a spur-of-the-moment email, one-upping someone else (I’m not sure, but it might have been the econometrician N. Scott Cardell), later posted on an economics-jokes website.
 
In the first-posted version, complete with misspelled Myrdal, it was:
 
“Economics is the only field in which two people can get a Nobel Prize for saying the opposite thing” is true, but is not strong enough. Better: “Economics is the only field in which two people can share a Nobel Prize for saying opposing things.” Specifically, Myrdahl and Hayek shared one. Roberto Alazar
 
Later, someone preceded it with: “Humor is evolving, now we have a refinement.”