UPDATE: a hardware kit is now available!  click here  
I am proud to present the Itty Bitty Double Extruder, a two-material belt-driven extruder with mounting hardware for MakerFarm i3v printers.  
Versions are provided for 3mm and 1.75mm filament.  
The extruder should be easily mountable on any printer that can accept a standard Hinged Accessible Extruder.  
Features:  
For Original Hexagon hot ends on 20mm centers (NOTE: NOT Hexagon AO)  
NEMA 14 motors  
GT2 belt drives with printed pulleys  
Integrated servo Z probe  
Cam-lock idler bearing pins  
Uses standard hobbed bolts available here
2mm narrower and 2g lighter than the stock MakerFarm extruder  
Includes mounts for Prusa i3v with integrated hot end cooling  
Discussion thread:  http://3dprintboard.com/showthread.php?8848-Itty-Bitty-Double-Extruder  
See the instructions for a list of vitamins.  
Note:  You will want to use firmware that will lift the Z axis when deploying and stowing the probe to keep it from hitting the bed.  If you're using Marlin…