several video cards and VRAM

i’m working with very large data to display.
Let’s say that RAM is full.
The display algo uses VBO to move displayed data to VRAM.
A first problem is that because i can’t have a precise idea of available AGP memory. If i send too much data into VBO, it swaps with RAM, and I don’t want that to happen. There are already many posts about that, and that’s not the point here. The user will have a preference to give a hint about what memory to use.
But i’m wondering about SLI or cross-fire cards. Is-it possible to have two 512Mo cards, and so having 1Go of VRAM ? And is that transparent for the program ?