Just been thinking and looking at my AV amp, seems I could run the flanking subs from the surround/surround back pre outs and it lets you have fronts small with crossover and surrounds large, you can set the appropriate delays and run it in 7 channel stereo mode.
I think this would work for a music only 2 channel setup with flanking subs and distributed subwoofers.
The problem comes with movies and the surround channels are coming from the front, so you would have to either switch the flanking subs off or do some cable jiggling pre film???