Just for your information : It is possible to run 2 motherboard with one PSU. If you can easily find cheap motherboard/gpu/ram that accept 4+ GPU (using pcie1x) then run 8+ GPU with one PSU.
I know that's not exactly what you aim for but it can be an alternative.
I wish there was a PSU with enough power to feed two motherboards with 8 GPU each. Usually the reverse is true: you would need 2 PSUs just to provide enough power to run 8 GPUs in one motherboard.
7950 card would take about 270-300W (mining scrypt), even if you have 5, it is already 1500W for GPUs only.
5970 card would take about 330w to run, so 4 of them connected to the one motherboard is 1320 for 4x5970 only.
Even if there was a PSU rated at 3000W, where would you plug it? You would need 30amp circuit to plug it in, and regular US circuits are only rated at 15amp, which is 1650W.