As far as I know there will always be two characters running around at the same time, if there is no one to play the second character the AI will take over controlling it, following you along as you play. You can switch between characters at will.
The way I've read the descriptions, this is only a single player game with two characters that work together, much like in LEGO: Star Wars titles. In fact, I've not seen the word "multiplayer" or the phrase "two players" at any time on the five sites I've been to where this game is featured. "Co-op at a distance" is the only thing I've seen. I find that to be an extremely weak marketing phrase, to be sure. It hints at being two player capable without committing to it.
I'm still searching for gamesas confirmation that this is indeed playable by more than one person. I'll come back and edit this, should I find that info.
EDIT:
Thank you, Hungry Donnor, for the link you provided below. While it's not an official stance, it would certainly be silly to allow link mode for a 360 console if two players weren't playing at the same time in the same game. 8) Why not advertise it as a one and two player game instead of "co-op at a distance"? That's rather like "eating at a distance", is it not? You either offer co-op or you don't.
On the other hand, if that is BS's way of saying, "you can go where you like while playing within the same game world," then I am hoping that at least one of these dev interviews will go into that. It rather becomes a (non-massively) multiplayer online game at that point, doesn't it? MOG, rather than MMOG as an initialism. Having one to eight players in one large game world and being able to go where one pleases is what I've dreamt about for Daggerfall since my first day playing it.
If this is a taste of what's to come with the next in the TES series, I am over the moon - but I'll still buy it if it is single player only, of course.
- Nasotha
P.S. I didn't mean to imply that in the LEGO games, you weren't allowed to switch characters at will. I see how it could be read that way, though.