This may sound totally dumb right now but I've thought about it long and hard and decided that if they don't fight they are at least going to run into each other and soon.
Reason 1: They are both musicians. This is not a very strong reason but Chopper fought Hogback because they were both docs, and I would like to see a musician face off.
Reason 2: The people who tried to conjure up Satan and ended up with Brook have been battered and beaten up by long-arms. Apoo is a long-arm.
Call me crazy, hate on this thread but don't rape me. Or say my idea is impossible, I won't like that. Unless you have good reasons then I'd love for you to go ahead. But I still don't want you to rape me.