Printing multiple materials at once (no replies)

Hi guys,

I'm new to the forums, and have only just gotten into 3d printing. I am nearly done with my makerfarm prusa printer and am just about ready to start building stuff with it. My interests at the moment mostly involve printing multiple materials at the same time. Has anyone done this with a reprap printer before? I'd be very interested in hearing about what the specific engineering challenges involved are? It seems like getting two extruders running in parallel is a difficult but solveable problem, I'm just not sure what other things I'll need to be thinking about.