wait a sec...are you planning to ground the amp to the battery terminal or to the chasis of the car? and you connected your two batteries in paralle? ok you're seeming to confuse yourself with all these spliters and junk. Tell me exactly how many amps your running.
Cuz if this is the case...just go return all those spilters you bought. if u have 2 amps that require a 4 guage power, go buy a spilter that splits into (2) 4 guage. Run one terminal to the farad cap, which that will go to your highest output amp. With the 2nd terminal, power the 2nd amp.

i don't know man...very confusing what you're trying to say!