I think Pixel Junk Monsters is the only tower defense game on any console (that I can think of, anyway... unless you count the WiiWare game "Defend Your Castle", but that plays nothing like PJM).
I know there are a couple tower defense games coming to the DS. NinjaTown and some arctic themed game where you fend off penguins and stuff (looks a little lame, and is also coming to Wii).
Your best bet, for now, would be the
many free web based tower defense games, but none of those offer coop.
I should mention that, if you have the expansion for PJM, and you beat all of the levels in the normal game
and expansion, then you'll unlock a random stage generator, so you guys can keep on playing "new" stages, even after you finish the game.
I don't think many people know about this feature. My fiance and I found it after we beat all the stages in coop. We're also huge fans of the game.