Yeah i would say twin batteries too.

I ran i a twin battery setup in my old Lancer ( Was doing car audio comps at the time), and to make them charge i ran a rather large relay ( can not for the love of me remember what size it was). I ran a spare accessory wire from the deck to the back, and used that to turn the relay on.

had a small sealed battery from a golf cart in the back layed down. Worked a treat, don't think i ever got voltage drop. ran 4g from front to back battery, then 0g from battery to amp.

This was only for a 2x200wrms setup ! But, it sounded perfect, and back in '02-03, was one of the few cars down here that ran it.. scored 138.8db off 2 x 10" Clarion Subs.. Box design ( as most installers will tell you) is always a winner.

Pete