How does Vram work for gpus in sli

I have a 780 with 3GB of vram and i heard that vram doesnt stack so a 3GB card and another 3GB card would still only give me 3GB of vram. Would that mean that if i buy a 780 with 6GB of vram and use it in sli with my 780 with 3GB would it give me the 6GB of vram or does it just go of the weekest one?

It would be the weakest as the VRAM has to mirror each other on both cards; so you'd only have 3 GB of VRAM.

Alright thanks for the quick and helpful response 

indeed vram will never be doubled with 2 cards, also buying a 6GB gpu and pair it with a 3GB gpu you still will only have 3GB usable.

* Not sure* New crossfire from amd might offer vram i/o mapping through dma channels.