How would a FPS/RPG coop differ that much from a game like Neverwinter Nights (I've never played it though)?
System Shock 2 was a FPS/RPG, and it had co-op. I didn't like SS2, though, so I'll recuse myself from any commenting on whether I thought that worked or not.
However, the Baldur's Gate and Icewind Dale games were pure RPGs with co-op, and I thought it was a completely worthless in this case. The fun part of those games isn't controlling your little dudes in combat, and you couldn't split up effectively, so it just didn't work for me at all.
This comment was edited on Aug 9, 19:07.