I haven't seen dual 8700s, but I have seen dual 8600s in several Toshibas. Either way, I'd want the single GPU for a variety of reasons... heat, battery life, compatibility, etc. Assuming the title fully supports an SLI setup, the two should be fairly evenly matched... but that's a fairly big assumption to make.
That Gateway with the 8800 GTS keeps tempting me (the one that is generally on sale at BestBuy for $1249) but there are just too many compromises with that system.