Mainly talking about dual-SLI here for consistency. With past DirectX (and OpenGL) APIs, VRAM was mirrored across graphics cards. With dual-SLI, this was possible by rendering one frame with one graphics card and another frame with another one. There was also a rendering option in which one graphics card would render part of the screen. Unfortunately, there doesn't seem to be much technical information on how VRAM stacking would work. What possible techniques could graphics cards or DirectX 12 be using to allow for this?
In DX12, there is no implicit driver-implemented SLI as there was in DX11. Instead, multiple GPUs are exposed to the application as separate "nodes" within a single DX12 device, and each VRAM resource lives on a single node, specified at creation time. There is no implicit mirroring of resources to both GPUs as in DX11 SLI.
So, the game engine or application has full control and responsibility over how data and work are distributed between the GPUs. It's up to the app developers to implement patterns like alternating frames between GPUs, or splitting the frame across GPUs, if they wish. For example, to implement alternating frames, the app would have to allocate all buffers, textures, render targets etc. on both GPUs and populate both of them. Then, when rendering a frame, it would generate a command list for the current GPU using its local copies of all resources. Any inter-GPU transfers needed (for temporal effects, for instance) would also be the app's responsibility.
More information on this topic can be found on this MSDN article on DX12 multi-GPU support.