Quote Originally Posted by chew* View Post

If the distance between both GPU chips is shorter on X2 versus 2 cards there will be a performance boost, we see this in benchmarks already.

Shorter paths, less resistance = gain.
To the best of my understanding, the issue is not electrical resistance, the issue is capacitive reactance between data lines. Crosstalk between data lines on a pcb, is a function of distance between data lines, length of data lines, and the frequency of the data lines. While keeping the data line trace at a fixed width, cross talk will then be a function of length and frequency. In decreasing the length of the data run by co-locating two GPUs on a single card, one can increase the data rate (frequency) between both GPUs before crosstalk becomes a factor, thus failure. In all serious, at the very high frequencies of data transmission in todays PCs, mere millimeters make a difference.