Hey Chris
I'm not very multiplayer savvy but I don't know of any games similar to D:OS with the style of multiplayer you describe, though that's not to say it couldn't be done. However that's also because D:OS is quite unique. Was it possible to do what you describe in the main campaign of Neverwinter Nights? (I don't know I've never played it).
The main reason I see it being problematic in this game is that D:OS is designed around the evolving story of two protagonists, whose dialogue choices and actions affect their relationship (through mechanics such as affection and affinity) and the plot itself. Creating separate character files would require these mechanics to work differently, if that is even possible, and as I understand it would require the redesign of much of the game. The story is supposed to be about the journey of these two characters as they grow together over time and so the game requires their permanent presence. I suppose you could allow the addition of separate characters merely as party members (not protagonists) though I'm not sure this would be as rewarding as playing the main heroes and would also require the entire game to be re-balanced (in terms of character system and encounter design) to accommodate this possibility.
In other words the game is not designed to be played the way you suggest. It might be possible to include this option, yet it would probably be a prohibitive amount of work, not to mention adverse to the game's central concept.
