If you have someone willing to print for you why not try out the reliable standard of a Greg/Wade: [
www.thingiverse.com]
I've been working on my own design for a dual extruder setup using the herringbone gears from the above, and it works very well. Working on my own design has made me realise how refined the version above is. It just didn't quite fit my needs or my bone-headed tendency to want to try my own thing