The GTX 690 is a dual-GPU solution which means that it's two slightly crippled GTX680s glued together on the same PCB. Each GPU comes with its own memory.
When multiple GPUs are used during gameplay each GPU works independently either on the same frame (interleaved, but this is very, very old) or on alternate frames (most common).
Since sequential frames are often nearly identical to each other and usually contain the same scene elements, it stands to reason that when two GPUs are working on sequential frames independently, they will require most of the same information to render each scene. This results in the 2048MiB of GDDR5 on each GPU containing very similar data at any point in time. They will not be perfect mirrors of each other (as that would result in the two GPUs doing the exact same work, which would negate the whole point of SLI/CrossfireX) but there's a lot of overlap in that textures, models, geometry, shaders, etc... will be present in both of the 2048MiB blocks.
It is not possible for the two processors on the GTX 690 to share a combined 4096MiB of GDDR5.
VRAM is not shared between the GPU's. So you will run into the same theoretical VRAM limits of a single GPU with 2GB of VRAM, i.e, multiple monitor resolutions and high AA settings. I would only be concerned if you planned to run multiple monitors, and then only mildly concerned.
As you can see in this worst-case scenario (Crysis 3 on multi-monitors), going from a single monitor to triple monitors does upend the relative standings in favor of the 6GB Titan card, but doesn't really kill the performance of the GTX 690. In fact, the 690's 2GB VRAM takes the lead over the 3GB 7990 when resolution is increased in this example. The results remain consistent just looking at the GTX 680 versus the 7970 GHz, so we know it's not just a driver issue.
We see similar results in a less demanding game, Battlefield 3. The 6GB Titan gains the most in the relative standings. But again, the 2GB GTX 690 and GTX 680 cards maintain their positions relative to the 3GB Radeon cards, just a couple FPS behind at both resolution settings.
In short, GPU power is important and VRAM is not a "wall" or "cliff" that you fall off. It is a limit, but not the end of everything, and even in most cases it isn't even a major factor at all.