amplifier splitter cable

I have a Pioneer DEH-X3500UI (DEH-X3500UI - CD Receiver with MIXTRAX™, USB Control for iPod®/iPhone®, and Pandora® Ready | Pioneer Electronics USA), which has 2 RCA outputs. I am currently using one of those for my Subwoofer amp input. I want to put in a 600 watt amplifier for my door speakers...