hahahahaha no...a cap is NOWHERE near a useful replacement to batteries. not even close. all you gotta do is wire them in parallel to the front battery. i'd use 1/0 gauge at least. you can ground the batts in the back, tho you'll see better performance if you ground to the original battery. make sure to fuse them as well. 18" from the front battery and 18" from the back.
You would probably be better off to invest in a good capacitor or two, that is if this is for a sound system. The weight of two batteries compared to two capacitors is alot.