I tried my best to minimize the weight penalty over many non-bowden single extruders. I believe I can have a working unit under 500g. The first working prototype is 600g, including hot ends and motors. Many direct drive single extruders are in this weight range, making the weight penalty of a second direct drive extruder negligible.Care has been taken to allow for flexible filament use, a straight path with minimum seams & joints, and no room to buckle.Filament idler pressure is easily adjustable, and there is a "bird nose" quick release to allow visible access to the hobbed pulley or to quickly switch materials.I designed around an Ord Hadron carriage, but with a simple mount plate, it will also mount to a MendelMax 2 carriage using a different groove mount plate, see my other designs in my account.This is a work in progress, but I have had about 40 hours of successful operation. The photos represent my first prototype, the drawings represent improvements based on use.Videos of the design running:https://www.flickr.com/photos/34962649@N00/14809349302/in/photostream/https://www.flickr.com/photos/34962649@N00/14576061370/in/photostream/On an Ord Hadron, it requires no modification to the carriage, and only a little adjustment to the X limit endstop. It still allows 196mm of full dual extrusion X working width. Mount plates for other machines should be coming soon.