Originally Posted by Fabrik8

Well, you have one set of preamp outputs, so you have one left and one right. So no matter what, you aren't going to have the ability to fade from front to back directly from the head unit. You can control the front/rear volume with the 4 channel amp though, so no big deal.

The 4 channel (Alpine) amp doesn't have a switch that I can see to combine the front and rear channels inside the amp, so you will need to get a splitter cable. What this will do is split your one pair of preamp outputs from the head unit into two pairs of outputs to go into the amp.

The 2 channel (Kenwood) amp can then be connected to the preamp outputs from the Alpine amp.

If you want the ability to fade from front to back, etc., from the head unit, you have two choices: You can either use the speaker level inputs to the Alpine amp, which won't give as good sound quality as the preamp inputs, or you can get a head unit with more preamp outputs. This is probably the better choice, as you should be able to go one or two models up in that head unit brand for only a small amount of money and get the outputs you want/need.