Subs aren't designed to produce directional audio, so you have to worry about phase and things.

Integrating 2 subs is about 10 times as hard as integrating one, because you have to worry about the other. There could be troublesome regions where the bass from both cancel each other out and so on -- and that includes reflections off of walls.

I tried that (in the following system: Spendor S3/5, Sunfire True Sub Jr., Musical Fidelity A3^CR pre and power), and I ended up only needing one sub, despite how not low the Spendors went.

Short answer: I wouldn't recommend it.