Bosom buddies makes sense because it was already a very common expression before the show was ever created. Bosom in the original phrase "bosom buddies" does not necessarily pertain to breasts. It's just an expression to mean that two people are close friends. The two main characters in this tv show just happen to be close friends who pretend to have breasts from time-to-time
Many tv shows and movies use common expressions for their titles and they're often meant to be a "play on words" -- like "Full House" is a term used in poker, but means something else for the tv show title.