Buying extra amps won't help. He has enough amplification, he just needs to get the signal there, which means more RCAs.
See if there's a different harness for your HU that has more RCAs. My ancient blau HU came with a single pair, but the CD changer wiring harness included a second pair.

 

i got a system in mine but i dont have the amp in the spare tire well, should i put it there and where should i bolt it in to?

 

I removed my spare tire because I now have 16inch wheels. I built a triangle shaped box and bolted it to the spare tire mount, then screwed the amp to the wood. Even painted the box red.

I think I have it hooked up best for what I got now and if I come across a HU with two sets of RCA's I might jump on the wagon.

Thanks again.