You'll pardon my saying so - but (assuming I'm following), you're saying you have two cards with 6G each and because of Crossfire, that gives you 12G video ram?

With all due respect, I'm pretty sure neither SLI nor Crossfire actually yield double the memory because of using two cards. This is a common misconception, in fact, and discussed as such all over online. Here's one source, there are many:

"A common misconception is that you will get double video RAM with double cards. In actuality, they mirror the data in the buffer. If you’re using AMD CrossFire and have graphics cards of two different VRAM capacities, then it will default to the lowest size. For example: If you have two video cards, one with 8GB of RAM and one with 6GB and they’re both compatible to be used with each other, both cards will act as if they have 6GB instead of 8 and 6, respectively." (LINK)

I think there was discsussion long ago about Dx12 being able to "stack" VRAM, but I believe that never actually materialized. Maybe I'm wrong.