There's a bit in Outlaw's SMS-1 guide specifically about stereo subs and the SMS-1 (under tips and tricks, page 28, under "deciding when to get multiple SMS-1's"). However, from what I'm seeing in the D2 manual (page 20, specifically), it's not really a concern in this case because the two sub outputs on the D2 do not appear to be stereo. There's a place in the speaker setup screen where you can specify "1 sub" (a single subwoofer that gets bass from small speakers and the LFE track), "1 super" (a single sub that gets bass from both small and large speakers as well as LFE - the same as the Model 990's "L/R+Sub" setting), "2 sub" (the same as "1 sub" except that it reduces the level at both outputs to compensate for the additional subwoofer), and "2 super" (same as "1 super" but with the levels decreased). In that case, you could simply set the D2 to "1 sub" and connect a single sub cable to the SMS-1. You could then connect both subs to the SMS-1's outputs.