As I remember car batteries have something between 20 and 40 amp-hours - depending on what car you have, what brand you want (and on how much it costs - I guess a Wall-Mart isn't too expensive)

I've done things like that (My car made the music for parties), too for at least more than 5 hours - But I had only 1 12" sub to drive not 2 of them

TeeMcBee