Sharing NPCs is just fine. :) Please feel free to make him/her/it/them do whatever you wish. 

It's funny, I've definitely thrown a wrench in the works at times, completely by accident, merely because the character's driving force insisted they respond to something in one way while my partner wanted a different response. Then again, I've played to others' hopes and intentions and ended up losing the character completely (it happens in particular when I've got to get a character to fall for another's without good cause - turns everything two dimensional). Therefore, I suppose I'm a firm proponent for following your own sense of story. We'll thread things together and make a new weaving which neither of us knew was possible. Some of the greatest shifts have come from that. Do let me know if something I do mucks up plans you had or pushes your characters in places they don't want to go. I can always, always pull back and redo a post to make it easier for the both of us. 

As for everything else, I find myself writing out bare bones with a lot of flexibility so that my partner can co-create. Much like improvisational theater, provided we build on what the other has set out, we're going in a good direction. 

And please continue to question whenever you'd like to. I'll be as honest as I can be, so that you can choose. Unless I'm in direct control of an action (in which case I'd write it out in post) I'm pretty flexible, I hope. Heh. So even if I'm throwing out ideas, do know that they are mostly just brainstorming or trying to put words to a somewhat tenuous idea which may or may not be something else entirely given what you'd like to see happen. I suspected you had something of a plan for them to be stuck together and thought you might utilize an NPC to make it happen if necessary. Either way, it was FUN to put a face to the magic of the pond. ;)