I think if one character is in trouble and needs help, it would be expedient (especially playing co-op) for the other character to go help before they got caught in a fight themselves, or were thrown in jail, etc.
I don't really see this as a problem. How many people playing single player are going to want to split up their characters and play each of them separately? How many people playing co-op are going to keep playing co-op if one gets into trouble and the other ignores them indefinitely? I suppose a time limit on each turn would solve the issue if it is determined to be a problem.
