A real stereo sub setup requires you to run the full range for the left and right channels to each sub. Not many processors have true left and right sub outputs. Even those with 2 outs are almost always a sum of the left, right, and LFE channels. So you'll need to set your processor to "full range" for the main speakers. Then you'll need a way to crossover the bass frequencies for each channel and send the mid/highs to the speakers and the bass to the subs. That means using the crossover in each of the subs or using a stand-alone crossover.