I don’t think you will find any information about “combining” functions. This is someway unique for the Navio2 boards. It is a hardware limitation of a chip that we use.
You just have to have the same frequency on channels 1-4, 5-8, 9-12, 13-14 and you will be good to go.
Here is some information about the Stretch update, which fixed the 5-8 issue: Raspbian Stretch Release for Navio with ArduCopter-3.5 and ROS Kinetic