Why do you have 2 seprate 12" subwoofers?
It started as just an experiment because I had both subs and wanted to see what would happen. The result was simply awesome bass response. The room I'm in has a particular room mode that spikes frequencies around 40hz. Adding the second sub provides another source of low frequency acoustic waves that seems to attenuate that spike. Combined with two simple corner traps to further attenuate the spike, the result is astounding. I feel enveloped in relaxed, natural, yet hard hitting and prodigious bass. I can't go back to a single sub.
Do you set one of the subs crossover really high to play upper bass and the other really low to play lower bass?
Both subs play identical material split from a Y adapter. It's the interplay of bass waves from the two sources that cancel and blend with each other to produce a more uniform response over the low frequency spectrum.
At least that's the way I understand the technical aspect of it, but there's no denying it works. I can't understate the importance for you to consider treating the room you're using with a couple of simple corner traps. If you have a square room, you're setting yourself up for the worst possible situation for dramatic room modes and standing waves, especially when you choose a sub with deliberate overkill.