term:

Wharton's Rule

wharton's rule n

[after Francis Wharton (1820-89), American lawyer and author, who formulated it]
: a rule that prohibits the prosecution of two persons for conspiracy to commit a particular offense when the offense in question can only be committed by at least two persons NOTE: Wharton's Rule does not apply when legislative intent is to the contrary (as when the legislation imposes a separate punishment for conspiracy to commit a particular crime).

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.