Running two subs with the 990 would not be a problem at all. I'm not entirely sure about spreading the bass out - one premise integral to the concept of the subwoofer is that very low frequencies are fairly non-directional (can't be pinpointed as coming from the single source), but you would be more likely to get a flat frequency response by using two subs. Either approach (one sub or two) works best with some care put into the placement of the sub(s).

Both subs' power ratings are based on the amps provided with them. The LFM-2's amp is rated for 150W RMS with the ability to provide up to 800W for brief peaks. The LFM-1's amp is rated for 325W RMS with up to 1300W for brief peaks - necessary because of the larger driver in the LFM-1. As for how much power is actually output at any given time, it is of course going to be dependent on the source material (how much low frequency material is present and how loud it is), system setup (where the sub amp's gain is set and where the processor's sub trim is set), and the volume.