Great suggestion! Haven't thought about this trick and save $100 on the MiniDSP - just thought the (2x4) would make it easier / more accurate in setting up two subs. The (2x4) also has the ability to add some PEQ filters and/or a low shelf to taste.
Thinking aloud about the general set-up method (after sub placement has is decided) for multiple subs being:
(1) Set the phase angle for both subs to zero then gain match the subs first.
(2) Run Audyssey to determine the distances of each sub individually.
(3) Get the delta between the further away and nearest sub. This will determine how much delay is needed at the selected crossover.
(4) Add some delay (phase) to the nearest sub so that it can be time aligned with the furthest.
(5) Re-run Audyssey with both subs to get the combined response for level and distance.
(6) Check with REW that low frequency response is flat. Check also that the crossover is smooth.
Maranatz NR1504, Oppo BPD-93, MTV 7000D, LG LW6500, aTV, WDTV Live, Harmony 650 remote, KEF E301, MiniDSP(2x4), Rythmik F12G