I'm gonna call a 980 ti X2 or Titan X2 both with dual Maxwell GPU's 980 ti X2 would have 12gb of GDDR5 and Titan X2 24GB of GDDR5
Probably not..
They may make a 980 Ti dual card, that has "12" GB of GDDR5, and by that I mean each GPU has access to 6, but SLI doesn't "stack" VRAM between GPUs (even on dual GPU cards), so you have two GPUs on one circuit board with access to only 6 GB of VRAM each for a total of 12. GPU 1 can't access the 6 GB belonging to GPU 2, though. (This is how it currently works even on the "12GB" Titan Z)
They definitely won't make a Titan X2 with 24GB of VRAM. Reason being is they already ran out of room on the PCB and had to put some of the VRAM modules on the back for the Titan X to fit 12GB. They simply wouldn't have enough room on the circuit board to double that, AND keep heat manageable (those back VRAM chips get up to 100C)
And, just the same, if it WERE possible, it would be each Titan GPU would have access to 12 GB, not all 24.