run a splitter I had the same deal with my jvc kd-avx77 they said it only needed one but with one I only get half the output so I ran a splitter and the difference is very noticeable
One of the inputs on your amp is a line out. You use it when you need to link another amp. So just hook the RCAs from your headunit to the "Line In" and you will be fine. Once again, the line out is if you needed to add another amp to your existing one.