How is the ownership listed? I'm not sure, but I think it would be like any other co-ownership situation--if it is listed as x or y, only one person needs to sign the paperwork, but if it is x and y, both need to sign all associated paperwork.
Sure. My mother and I co-own all of the minis, but we made sure to do it as x or y. We trust each other completely, and that way if something happens to one or the other of us, we don't have to worry about getting proper documentation, etc.