I would simply buy some Gold Subcription for the console. And let them run wild downloading demos. Then if they the next day are playing like crazy on a certain game, get them that one either as download or retail.
Two controllers and Gears of War split screen or fights in Bomberman is gold though. Actually if they are into action RPGs get them Sacred 2 (not a perfect game at all, but I still spent 200+ hours on it with my GF ages ago). All 3 games mentioned are cheap and can be played co-op.