The problem here is that you have the stereo sound production though. You'll lose that. The way the music is made on the cd's, the music is put through left and right, so you actually have what could be considered a sound stage. If you listened to it live, you would have certain parts coming from different sides - and the side to side reproduces that.
What it looks like you need is a 4 channel amplifier for the speakers.
And, if you are really set in using it - running the two channel to the front/rear is certainly possible. Won't damage a thing. It's just not good for the sound quality. What it will end up being is like listening the music sideways.
Or, alternatively. Run the back speakers off the two channel amplifier, and the front off the head unit - this will give you the power in the back, while keeping the stereo sound.
