merge 2 tracks?

Hi,

I'm doing a project where I need to split a video input into 2 tracks:
foreground and background. I think I can figure out that part. But, I'm
not sure if Xvid can compress the 2 track at different bit rate, then
merge the 2 tracks into 1 mpeg4 file. I've been reading the source code
for couple days now, but don't see a way to do it. Does anyone know?

jian