I haven’t seen anyone trying dual extrusion on a belt or anyone discuss the the immediate issue I see with dual extrusion is it could require multiple tool changes during a single layer which would significantly slow print speed. Other than that I don’t know how it would be affected. I think we could benefit from using dual extrusion mainly for support material or for a raft, so as to not print directly onto the belt.
What if we used multiple XY gantry on a belt, one gantry placed at the beginning of the belt printing a raft and supports a few layers ahead of the extruder for build material. The 1st extruder for support material would always start first and create the support structure which the 2nd build extruder would print onto creating an endless support structure. Just an idea.