well if you don't care about the ability to have fading (forward/back) and just stick with stereo then I think that you can just put the front speakers in parallel with the rear amp (i.e. but both '+' to the '+' on the speaker and both '-' to the '-' on the speaker). However if the 2 signals are even slightly out of sync then it might really screw up your speakers.
Honestly I wouldn't bother doing that. What HU do you have and what speakers do you have and what amp? You'll prolly blow your speakers trying to do that