And why not that third idea ?
Let's say you're playing single player with the both main characters and one of them fall in a deep pit (of a troll or something big beast/monster) ... then the other one can go find or buy a rope and rescue the other before the troll is coming !
This can be part of "an adventure", why not? And if this is possible, why not rhiggins idea of a kidnapping?
Of course, if your character can handle kidnappers and kill them, no kidnapping afterwards ...
I also like to see some riddles/puzzles in the game you can solve with 1 character, but you can solve in a much faster/easier way with both main characters! Then the chooice is yours ! Maybe even a few puzzles you can ONLY solve if you play well together, otherwise ...