Green’s Dictionary of Slang

(US black) to obtain two dollars’ worth of something.

1946		Mezzrow & Wolfe Really the Blues 218: Hey buddy ghee, why don’t you put it with me and we’ll cop a deuceways.
1959		J.E. Schmidt Narcotics Lingo and Lore 34: Cop a deuceways – To purchase two-dollars worth of narcotics.