several video cards and VRAM

Hello,
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 ?