There will be a 280GTX x2 alsol which I would believe will be the fastest GPU, but w/e I think the 4870 is a way better deal.
1. Cost: The cost for just a single GT200 chip is enormous. Adding two to that equation and even if they did do it, nobody would buy it and they wouldn't be able to sell enough to get back the manufacturing costs.
2. Size: GT200 is a big chip, slightly more than twice the size of a single RV770 IIRC. There will be issues just trying to fit two of them, and their memory, and all the connections required for the 512-bit bus, and the PCI bridge in the same card.
Sure a 55nm die shrink could help with all of these, but I think the signifcance of the 65nm->55nm in the GPU business is overrated. It's not the huge leap that going from 65nm to 45nm is in the CPU business. Just look at the marginal gain from 9800GTX->GTX+ and then don't really expect a whole lot more for GT200b.