Newb and have to write a custom mixer

First of all
I’m new to this whatever community it is called (please tell me)
I’m building a synchropter or an intermeshing rotor heli using 2 sets of CCPM.
So I pre ordered a Pixhawk 2 seeing it as the best flight controller in the long term that I can put random mixer into.

While waiting for delivery hopefully next month, I want to know how to get or make this mixer.
It would be great if someone can share his work but in case I have to write it up myself- where to start?

This should be a decent start (if not exactly what you’re looking for):

I think I will need a newbier start.

EDIT: after some playing with git, and my new PH2
I tried fhedberg’s code, modified a little to suit my heli and now I got desired servo movement.

However, manufacturer recommends flashing latest firmaware. Until his PR get merged, am I risking using 6 months old code with my PH2?