What do you mean when you say "2 guys"?
Do you mean a party system where you have a group of characters?
Or are you thinking of multiple story lines that interweave each other?
Also, what type of choice are you giving to the player?
I mean developers
Party system isn't part of topic exactly really.. sure you have evil and good party members but it kinda.
Game can be equally good with or without a party system.. depends on the game.