well first of all they're listing VRAM in an extremely misleading way. YOu're only getting 4GB with dual 980s and 12GB with dual Titan Zs, not 8GB and 24GB.
The most common form of SLI works by splitting frames between the two cards; one card renders frame 1, the other frame 2, etc.. To do this properly, they need to share video information and what's called the "framebuffer"; to do this, both cards are given the same information in vram. By doubling up on GPUs, you don't magically gain an additional 2-4GB of VRAM. You only have as much VRAM as each individual card has available.
DirectX12 and other low level APIs will be changing the way framebuffering works and will allow VRAM stacking, but that would only be for DX12 applications.
Just get the 980s anyway. Don't go above 2 GPUs, it's inefficient and a waste of time/money. Don't try to render videos and play 3 games at the same time on the same machine, no CPU/GPU combination will be able to do all of that at maximum speed.