It's an interesting question, and there are a couple ways to look at it. Probably the most imporant factor is whether or not your two subs are truly in a stereo configuration (each getting a different signal).

Two subs that share a common signal are a simpler case to consider. On the one hand, using two SMS-1's would let you equalize each one separately so that each had a smooth response and each had its phase adjusted as optimally as possible. On the other hand, the overall system response is the underlying goal, and a single SMS-1 can pretty reasonably achieve that with two subs sharing a common signal.

If you truly have two stereo subwoofers (meaning each gets a separate signal, one paired to the left channel and one paired to the right channel), you would probably want to use two SMS-1's because the SMS-1 produces a mono subwoofer output. Using a single SMS-1 would require you to sacrifice the stereo signals.