Who said 2 cards in SLI doesn't double the memory?

There was a very knowledgeable person on these forums who showed me an article he posted about SLI and in that article it states that having 2 cards in SLI does NOT double the amount of available memory, only one card's memory is used.

So how come when I play GTA V it says my available graphics memory is 12 GB? that's two card 2x6GB so is it really making use of the 12GB or what's going on? I'm confused