You are wrong. The Titan Z is two Titan cores in SLI, so it is the exact same thing. 6gb VRAM per core, and SLI drivers.

Vram and SLI do not work that way. Information loaded into Vram has to be duplicated 1:1 on both cards when in SLI, multiple cards with 6GB Vram each still only has 6GB of available Vram in 2-way, 3-way or even 4-way SLI.

The Titan Z has 12GB of available Vram, it isn't split between the 2 PCBs, the Vram is seperate from the individual PCBs and accessable as if it was a single card. I was wrong about the SLI part (up for 36 hours straight helping a friend move) but not about the Vram. The Titan Z would still have to run on SLI because it is running 2 PCBs but the Vram doesn't need to be split as it is joined in the same memory bank, not split between 2 seperate GPUs.

Think of it this way,

2 GPUs in SLI,

PCB>Vram
^
SLI bridge
^
PCB>Vram

In that instance, information needs to be duplicated over the 2 GPUs because the Vram is split.

Single GPU, dual PCB,

PCB>PCB>Vram

Still SLI because of the dual PCBs but only a single Vram bank, so information doesn't need to be split.