Yeah, what vbigiani said. You don't HAVE to replace Imoen, I just figured it might be cool to take over her place in the story for a co-op game. Truly, you could play as three friends from Candlekeep (instead of just two), if you want... or whatever. The plot in BG1 is all about the protagonist, role-play encounters might behave differently based on who is in the party, but everything is doable regardless who is or is not in the party (correct me if i am wrong, anyone, but I can't think of a single quest or area that requires an NPC be present in the party for BG1).
Canonically, Imoen is in your party at the end of BG1 (because she is there at the start of BG2), but there is no real reason you have to take her, or anyone else around. I think that BG2 plot might work a little better if you already have an emotional connection with Imoen, but it is hardly necessary. I mean, you are going to understand the plot either way.
I think I may have made it seem more complex than it really is. You asked if you could have fun and understand the plot if you played a co-op game with your friend. I think the answer to both is 'yes'. Everything else I wrote was me spit-balling how *I* might go about playing a co-op game.
ACDude800: So, essentially, it works like other BioWare RPGs and I don't have to have a particular set of characters for the story to make sense. Okay, thanks to both of you for the help.
Sorry if I seem a little stupid regarding this; I've never played a D&D RPG before and it's pretty intimidating to me. Just want to make sure I have all the facts before diving in.
No problem, sorry again if i confused the issue with my first response. I think you 'get it' now though, so at least there is that :).