I'm not sure if one could call of the current dual GPU cards "dual cores" as they aren't really a dual core, since both GPUs are seperating and not built on the same die...these would probably be better compared to a multisocket motherboard as this is basically the same concept, two cores on one PCB but still seperate
I assume the simple fact why dual core GPUs are manufactured yet is the fact that rendering instructions aren't multithreaded and game developers are probably reluctant to do so already having to write multithreaded CPU code which takes a lot of time
I agree this is a step in the right direction from SLI though, dual cards offer I think 50% increase over single cores but don't quote me on that...they still aren't worth the price IMO