Dual GPU in current designs = duplication of memory with all the added draw backs of board complexity, power, cost, etc.
I don't think it is a horrible idea for the reasons given (I have echoed those long ago) but it does pose hurdles. If they do go with a SI maybe they could invest in cross-chip traffic/memory controller for shared memory. Then going with 2x130mm^2 chips has the benefits of manufacturing. That said there could be structural losses within the GPUs like schedulers and whatnot that would be duplicated and dedicated logic to get the cross-GPU to work. I wonder if it is a dedicated chip design a memory controller / side port communication could be efficiently worked out to minimize such issues.
Part of me says another options, if it could be pulled off, would be to use the PC market for binning of usable parts:
10% Top Bin = $500 tier PC parts / no defects, best speed/TDP bin
11-50% Bin = Xbox 3 Bin / Mid-PC bin, 80% frequency, 10% block disabled
51-100% Bin = Mid-Range & Low-End PC Binning, various disabled blocks, reduced frequencies
This would only be helpful the 12-18 months until new DX models come out. But if MS could coordinate this with a chip maker it could be a boost to the chip maker, "Game GPU as Xbox 3" or even better "Faster Xbox Chip". And while it would only be helpful the first year it would allow more usable parts--so even if they are sold at a loss to MS (lets say ATI doesn't need 1M extra parts at $40 chip, so they buy them at $30, BUT MS loses $10 instead of $40 on the unusable chips for the Xbox) they could come ahead until the processes matures to the 80% usable rate and/or the next process reduction. If course a PC compatible part is going to be larger than a console specific one.
I bet there is a lot on the table...