I really don't feel like writing a comic but I'd love to draw one, and about a year ago I asked a then good friend to write it for me. I took one look at the characters she came up with and her first comic, and I said as tactfully as possible, "I don't think this will work." Yeah, we aren't friends anymore (not because of that, though).
Basically, I think it is hard, because he's gonna wanna change things you write and you'll want his art to match your vision, probably. The only thing I can say is, try to be flexible, and good luck.
Yeah, I'm giving bad advice, I think Sideways does his comic with some other people (and his is really good!) so he could probably be a lot more helpful.