You can't add more vram, but I can tell you what's happening.
When you run SLI/Crossfire or have multi GPU video cards. The vram isn't pooled into one set of memory, it's duplicated for each GPU.
2 1GB cards in SLI doesn't let you use 2GB of vram. You still have the 1GB limit because each GPU has a copy of the memory in use.
A standard GTX 295 has 2 GPUs and 1792mb of vram or 896mb per GPU. So in your case it's a 4 X 896 setup since you have 2 GTX 295s.