There is a lady called something like Justine who built an amazing, large, machine capable of printing 4 copies simultaneously using Duet boards and who posted a year or two back on these forums. It used an X shaped gantry with an extruder mounted on each of the 4 points of the X. As an engineer, I can appreciate the difficulties that had to be overcome to get all 4 nozzles at exactly the same position relative to the (very large) build plate (in the Z direction). Using a single Z axis, also meant that the build plate had to be flat and level without using firmware compensation when using 4 nozzles which would be at different XY coordinates. The advantage of her design meant that simple kinematics such as Cartesian or CoreXY could be used because the tools were not strictly independent other than extruder motors.
