Actually, Atlus only brought over the first two GameBoy Advance "Swordcraft Story" games; the third (and I think the best one) never made it over here
Even so, if you can find Summon Night Swordcraft Story 2 for the GBA, pick it up. It's light years better than the first one (which was fun, too, but nothing compared to SNSS2/3).
Anyway, to get back on topic, the aforementioned GBA games in the Summon Night series are action RPG's (battles are fought as if they were weapon-based fighting games -- Tales of Phantasia "lite" in a way), whereas other games in the series are more strategic RPG's. This particular DS SN game uses touch screen combat. You can switch between both main characters and do physical attacks, magic, and other attacks.
I recommend checking out the
official game page (in Japanese) if you'd like to sample the game. Click on the bottom-right button (right under the circle), then click on the little button with the "x" in the bottom middle. You can now play a mini version of the game using both characters. It's pretty cool, and works really well.
In fact, after reading this thread and trying out the online demo, I'm gonna pick this game up
The FAQ at GameFAQs was pretty straightforward, and for $20, what do I have to lose?