VRAM stacking question

[TigersGhost]

Ok so I do know that if you have 2 2gb cards then you don't have 4gb. 

I am wondering out of curiosity, when you do have 2 2gb cards, where does those 2 gb go?

Do they get used, wasted, mirrored or what?

And so when I have something that uses VRAM like lots of AA or very high resolution, Do I gain by having 2 cards?

If I have a game, want to run at... say 3800x1800, MSAA x8 and lots of 4k textures. 2GB is definitely not enough. But if I had 2 or 3 of those would it help?

 

While im at it, second question. If I do plan to run something along like that. Do you think I should get another GTX 770 2GB or get something like 780ti with more VRAM?

[FloRolf]

Yeah, its mirrored. The same information is stored twice. You gain performance by adding another in term of raw power but if your vram is full, its full.

 

Thats why its better to have a single 290/780 at surround/4k than having 3x 760.

[TigersGhost]

Sweet. Thank you for the answer. 

So it's basically like RAID 1.