Would it be possible to get the empeg to play two mp3's simultaneously with the output of the first mp3 going to the front two outputs and the second mp3 going to the rear two ouputs?

This has been discussed before. Unfortunately, it's not possible because the player's DSP only accepts a stereo signal. The EQ and fader are all done in the DSP.

It's a standard off-the-shelf DSP made for car CD players, you see.