This y adapter/piece/part is made for the 3Dator 3D printer. With minor modifications and a few changes in the Marlin firmware it should work with other bowden printers too.
The mechanism is pretty straight forward. One Filament is printed with one of the extruders. On Toolchange the filament is retracted till the other material can be driven into the hotend.
We made a video for you to see it in action:
For more detailed information have a look into our wiki:
If you already own one of our printers feel free to buy our extension set with all parts that are needed.
You want to know more about us and what we do? Visit us on http://3dator.com
Please note that this design is under a Creative Commons - Attribution - Non Commercial - Share Alike License
Make sure you have a good cooling system. It is critical that the tubes for the filament are perfect.