Your computer will always use the lower amount of VRAM because it parallel computes the VRAM from the PCI-e lanes. Stacking is not possible as of yet and even SLI'd it progressively renders every other frame with games etc......The only way I have found to supersede this currently is to make sure you only have the cards with X amount of VRAM needed. I use 2 GTX 980 ti's connected via an SLI bridge, but SLI disabled, it seemingly gets better results via Octane bench 375 compared to 355. When I put one of my extra 960's in with them it automatically reverted to the 2 GB of VRAM but did push my Octane bench to 425........So there are benefits to having more cards, but there are definite drawbacks to not using the same amount of VRAM. Now with DirectX12, theoretically VRAM stacking is possible, but it has to be implemented by the publishers of the software and firmware. Hope this gives a little more understanding.