Combining composers (camera feed and AR layer)

I’m using Three.js and ThreeAr for an AR project. What I’d like to do is to use post-processing on the Three.js scene. However, if I use the UnrealBloom effect, it blackens out the background of my scene, hiding the camera stream.

I came across this code, that shows how to include the camera image as a VideoTexture. That solves the blackening problem, but if I put everything in one scene the bloom effect affects the background video too.

Is it possible to make 2 scenes, one containing the background video that gets piped into a composer with a vignette shader, and one containing the actual AR scene, that gets piped into another composer with a bloom effect and then render this one on top of the first one?