If BF3 is as vram intensive as people say it is I don't know why people think it's impossible, just Skyrim and watching a show on my second monitor uses 1.8-1.9gb of memory (main monitor is 1920x1080 and my secondary is an ancient 1280x1024).
(The show in questions is 720p resolution, perfect size for the second monitor, and for reference that's only 100-120mb of vram used)