At the moment, there’s no announced games that support the use of two GamePads. You won’t even be able to purchase a second GamePad from retailers. Reggie Fils-Aime has assured people, however, that:
The reason you don’t see games with two GamePads [at this event] is the technical ability to make two GamePads work was delivered to publishers after they started this current round of development.
(This is actually consistent with a rumour we heard a long while ago.)
…you’ll see those two GamePad experiences at a later date; when those games are coming, that’s when we’ll make a separate GamePad available.
Of course, this won’t make the GamePads any less expensive. That might restrict the amount of two-GamePad experiences we see.