Current dual-gpu cards work by using load balancing between two gpu cores on a video card. The current designs on these dual-gpu cards don't have shared memory so each gpu core uses the required amount of vram per gpu core to render and load balance two scenes together seamlessly. Shared memory wouldn't actually give you any more ram, but rather better management of how the gpu core's could handle the provided available on board ram.
With a shared memory dual-gpu you could do more things with the same amount of on board ram essentially. For example card with 1GB shared vram and 1 gpu core could be using 256vram for one application while the other core was using 768vram for another application. Now with a 2x512MB even though the card has 1GB worth of total ram on board your restricted to 512MB per core so that same situation wouldn't be possible. Shared memory is more economical and flexible, but more complex to design which is likely the leading contributing factor as to why it isn't yet used in dual gpu cards. I hope that helps answer your question.