The magnetic shift thought is from "foehnsturm" I redesigned it for the Ultimaker2. My aim was to remain close to the UM2 layout, you can of course change this and add f.e. a total metal head as 2th head.
The primary benefit of the set up is that you recover most of the buildarea for single head extrusion and can easy set the 2th head away.
Another + is using distinct nozzle temperatures in the various heads.
The key problem is the leaking of filament from the 2th extruder. Using a prime tower as well as retraction does help a lot. In this set up the leaking will not take place on the printed part.
You will want right and left parts, simply reflect the pieces in your slicer.
find a job outline including directions here;
upgrade 25/5/#20152015: Added the "pick up" unit that I forgot in the first upload, additionally corrected this in the measure file.
Upgrade: added new variant of the docking corners that clamp to the casing, so no need for tape. (all the parts with the name beginning with "non-tape")