Well I sold my system to a friend down the street and he already has amp/speakers/hu. How would I install another amp into that system?

I've never done dual amps. Do you just split the 12v line one into one amp and one into another?

Also what about RCA inputs? Does is HU need to have 4 RCA inputs?

 

you need to get a distribution block for the power wire. if his hu does not have enough rca outputs, then you need to find a 2-4 channel splitter. you need to check and make sure the amps can be bridged together if they will both be driving the same subs/speakers.

 

Well one amp will just be driving the sub and another amp just the speakers. You have a link to a 4-channel splitter, I can't find one.

Nevermind, i took the guy to a sound shop and we got the distributor and the 2 rca splitters.

 

more importantly is to use the proper wire gauge for the amplifier's draw.
watts / volts (14.6 in a tib) = amperage

1000W amp divided by 14.6 Volts = 68 amps

your wire must be good for more amps then your amplifier will put out.

here's a nifty site I found, but it's really made to sell big wire to people. it's rediculous how large of wire they want you to use
http://www.streetwires.com/products/wireGauge.cfm

Heres a wire gauge table and a wire gauge calculator at the bottom. use power transmission wiring values. Do not excede amperage
http://www.powerstream.com/Wire_Size.htm