Y-splitters come out of the preouts, run 2 pairs or rcas from there, what's confusing //content.invisioncic.com/y282845/emoticons/confused.gif.e820e0216602db4765798ac39d28caa9.gif
Some amps also offer pass-through jacks directly for what you are talking about. Place your input RCAs in and run the other set out to your other amp. Then you only have one set coming from the deck, but it can supply 2 amps. I know rockford uses this.
a lot of them are low pass only though, so if you are running the RCAs to your sub amp and then to a 4channel for speakers it may not work, so make sure you find out first.