“When two men in business always agree, one of them is unnecessary” is a popular line that’s been credited to U.S. chewing gum company founder William Wrigley, Jr. (1861-1932). The Big Book of Business Quotations (2003) has this credited to the American Magazine (March 1920), but the Hathi Trust Digital Library appears to show this in the American Magazine in 1931. The quotation was popularized in the July 1940 Reader’s Digest.
 
Similar quotations have appeared earlier. “If two records substantially agree, one is unnecessary — if they materially differ, one must be wrong” was cited in The History and Origin of the Law Reports (1884).
 
 
Wikipedia: William Wrigley, Jr.
William Wrigley, Jr. (September 30, 1861–January 26, 1932) was a U.S. chewing gum industrialist. He was founder and eponym of the Wm. Wrigley Jr. Company in 1891. He was born in Philadelphia, Pennsylvania.
 
Google Books
The History and Origin of the Law Reports
By William Thomas Shave Daniel
London: William Clowes and Sons, Limited
1884
Pg. 31:
If two records substantially agree, one is unnecessary — if they materially differ, one must be wrong.
 
Google Books
The Reader’s Digest
Volume 37
July 1940
Pg. 55:
When two men in business always agree, one of them is unnecessary.
—William Wrigley, Jr.
 
Google Books
Executive Thinking and Action
By Fred DeArmond
New York, NY: McGraw-Hill Book Co.
1946
Pg. 20:
“When two men in a business always agree, one of them is unnecessary,” said William Wrigley, Jr.
 
Hathi Trust Digital Library
Comic Dictionary
(completely revised and enlarged)
By Evan Esar
New York, NY: Horizon Press
1951
Pg. 295:
unnecessary Needless, as in the aphorism that, when two men in business always agree, one of them is unnecessary.
 
Google Books
Speaker’s Handbook of Epigrams and Witticisms
By Herbert V. Prochnow
New York, NY: Harper
1955
Pg. 37:
When two men in a business always agree, one of them is unnecessary. William Wrigley, Jr.
   
Google Books
Before The Fall:
An Inside View Of The Pre-Watergate White House
By William Safire
Garden City, NY: Doubleday
1975
Pg. 99:
“When two men always agree, one of them is unnecessary” may not be original, but it was Nixon’s way most of the time.
   
Google Books
The Big Book of Business Quotations
By Editors Of Perseus Publishing
New York, NY: Basic Books
2003
Pg. 146:
When two men always agree, one of them is unnecessary.
William Wrigley (1861-1932) U.S. businessman and founder of Wrigley Company. American Magazine (March 1920)
 
Google Books
Book of Business Quotations
Edited by Bill Ridgers
Hoboken, NJ: John Wiley & SIos, Inc.
2012
Pg. 65:
When two men in business always agree, one of them is unnecessary.
William Wrigley, businessman (1861–1932)