“When Greek meets Greek, they open a restaurant”

“When Greek meets Greek, then comes the tug of war” refers to the wars of classical Greece; the saying dates to at least 1677.
 
The Americanized version has had different endings. “When Greek meets Greek, then comes the candy pull” is cited from 1898. “When Greek meets Greek, they pool their savings and open a fruit stand” was popular in the early 1910s.
 
“When Greek meets Greek, they open a restaurant or an ice cream parlor” is cited from 1916. The “restaurant” ending—reflecting the many Greek diners throughout America—is the most frequent satirical ending that’s been used since the 1920s.
   
     
Answers.com
Proverbs: When Greek meets Greek, then comes the tug of war
When Greeks joyn’d Greeks, then was the tug of War.
[1677 N. Lee Rival Queens iv. 48]
Two upright Postillions‥ were disputing who was the greatest rogue.—‘When Greek meets Greek then comes the tug of war.’
[1804 W. Irving Journals & Notebooks (1969) I. 69]
When Greek meets Greek then comes, in this case, an exchange of anecdotes about the deposed sovereigns of eastern Europe—in a word, the tug of bores.
[1926 A. Huxley Two or Three Graces 175]
One is reminded of an adage Erasmus used—Magus cum mago: ‘magician meets magician’—Greek, as we say, meets Greek.
[1979 M. A. Screech Rabelais iii.]
   
2 November 1898, Daily Kennebec Journal (Kennebec, ME), pg. 4, col. 3:
“When Greek meets Greek, then comes the candy pull.” The quotation isn’t strictly literla but it is suggested by the fact that Biddeford’s entire Greek population, which now numbers two, is devoted to the purveying of sweets.
   
6 July 1906, Lake County Times (Hammond, IN), pg.  8, col. 3:   
When Greek meets Greek nowadays the conversation turns on the condition of the banana trade.
 
24 January 1907, Mills County Tribune (Glenwood, Iowa), pg. 3, col. 7:
“What happens when Greek meets Greek?” asked the teacher.
 
“Dey ax each udder how’s de peanut biz,” answered an observant youth.
   
14 September 1911, Sandusky (OH) Register, pg. 4, col. 4:
When Greek meets Greek then comes the shoe shining parlor.
   
14 June 1912, Oelwein (Iowa) Daily Register, pg. 7, col. 3:
When Greek Meets Greek.
“When Greek meets Greek—what happens?” asked the teacher.
 
Wise little Johnnie promptly replied: “One says to the other, ‘How’s de fruit business?’”
     
9 July 1913, Newark (OH) Advocate, pg. 4, col. 3:
Times have changed. Nowadays when Greek meets Greek they pool their savings and start a fruit-stand.
 
28 September 1916, Des Moines (Iowa) Daily News, pg. 4, col. 3:
“When Greek meets Greek,” they open a restaurant or ice cream parlor.
   
Google News Archive,
19 February 1926, Spokane (WA) Daily Chronicle, pg. 1, col. 4:
They used to say that when Greek met Greek the fur would fly. Later this was revised to “they open a restaurant.” Now it they engage in a wrestling match.
   
14 February 1927, Olean (NY) Times, pg. 13, col. 5:
When Greek meets Greek they usually open a restaurant, but when a sailor meets a sailor they fight!
 
22 October 1929, Salt Lake Tribune (Salt Lake City, UT), pg. 19, col. 5:
When Greek Meets Greek—They
Don’t Always Open a Restaurant

 
Google News Archive
5 August 1938, St. Petersburg (FL) Evening Independent, “Cook’s Tour” with George Rector, pg. 1, col. 4:
The old saying was, when Greek meets Greek they open a restaurant, and I guess it’s that way with the Chinese.
 
21 August 1938, Los Angeles (CA) Times, “George Koverly Opens Up on King King Kashey at Olympic,” pg. A14:
When Greek meets Greek the old gag says they open a restaurant but when wrestling villain meets wrestling villain they usually open each other’s heads
   
Google Books
Everyday Life in the Age of Enterprise, 1865-1900
By Robert Harris Walker
New York, NY: Putnam
1967
Pg. 140:
When two Greeks meet they open a restaurant, the saying goes.
   
29 September 1967, New York (NY) Times, “2 Combos Blasting City’s Rock Pile” by Dan Sullivan, pg. 54:
“When Greek meets Greek,” an old saying has it, they open a restaurant.
 
21 August 1968, New York (NY) Times, “20,000 Greek-Americans Having Words Here” by Peter Millones, pg. 47:
It has been said that when two Greeks get together they open a restaurant; when three get together they start a revolution. Twenty thousand are in New York now, and they are having a convetion.
 
Google Books
What’s Right Now?
By William J. Jacobs
New York, NY: Paulist Press
1971
Pg. 1:
I might add that there was an old gag that every time two Greeks get together they open a restaurant.
 
Google Books
Ethnic Groups in American Life
By James Patrick Shenton and Gene Brown
New York, NY: Arno Press
1978
Pg. 320:
It has been said that when two Greeks get together they open a restaurant; when three get together they start a revolution.
 
Google Books
The Greek Americans
By Alice Scourby
Boston, MA: Twayne Publishers
1984
Pg. 46:
It has often been said that when two Greeks meet, they open a restaurant.
 
Google Books
An Amulet of Greek Earth:
Generations of immigrant folk culture

By Helen Papanikolas
Athens, OH: Swallow Press
2002
Pg. 92:
“When two Greeks meet, they open a restaurant; when three Greeks meet, they form a political party” was a much-heard expression during those years.