the rca connections are easy-- as long as you have a stereo signal going into the amp. if one of the amps has "passthrough" audio connections, you can use those to feed both amps
ie: if your fullrange amp has this, but the sub amp doesn't, connect the signal from the pc to the input on the fullrange amp, and run the other rca's from the "passthrough" connection on the fullrange amp to the inputs on the sub amp.
the power wire is a little trickier.
what gauge wire do you have run? i assume you just used whatever came with the sub/amp kit? if so, it will probably be too small of a gauge, so you will need to re-run the wire with a larger, correctly sized wire(now do you see why i try to run larger wire then i first need?!? ). just add up the fuses on the amps, and check some wiring gauge charts for the correct wire run/size. the best way to do it is to run a larger, thick cable near to both amps, and then use a distribution block, and a smaller gauge wire off that to feed the amps. the ground just needs to be at least as large as the power wire going into each amp.
like these charts: