Oh man, this sounds like quite a sticky wicket.
I've never tried anything like this before, outside of crossing my own characters into other stories of my own. But I have total control over them, so it's not really the same thing.
I think if I were to try borrowing/lending characters with my writing buddies, it would have to be very orhestrated from the get-go. Along the same lines as sharing universes with another author, only sharing characters would be much more difficult I think. It's gotta be a tricky task to take a character out of his/her native universe and transplant him into a new/different one. Depending on the variation between the worlds, you'd have to handle whether or not the character understands how the new/different world works, what the timeframe is in relation to the events that happened in the original story, etc... I don't even know how many different aspects would have to be dealt with for this to work. j
Personally I don't think I'd have the gonads to try it, nor would I be able to let one of my beloved, albeit undeveloped characters run off with someone else. That's the greedy, "this is my creation and I want to take all the credit" part of me. However, I'd be flattered if someone wanted to use one of my characters. I just don't think I'd let it happen, at least not without a long, detailed talk about what the other author had in store for my character. If I"m planning sequels, I don't want this other author's story to interfere with the timeline/plot of what I had planned.
As far as it being a good or bad idea, well I don't think this issue has that definate of an answer. In the hands of a very organized, prepared group of writers it could work marvelously. But I think it would be very troublesome if someone wanted to take an already established character and put him in a different world. For me it would be too complicated, anyway. My mind just doesn't work like that.
But hey, that's me. I can barely chew gum and breath at the same time without choking myself.
BLG