Setting up an Iris in trainer-student mode

We have sadly lost a Iris in 淡水河, the main cause is the weak GPS signal, it was raining that day, we do our test under bridge, GPS count still have 7~9, but the hdop is about 2.1.
It cause iris to shift in loiter mode. when pilot tried to pull back Iris, another sad thing is that he is a mode 1 flyer, and our Iris is a mode 2 one ....Iris fall into water in a panic.

We then have an extra controller... The idea is to make a trainer one, so that we can have a trainer to step in, in case of emergency.

First I have checked the channel setup on the instructor one, it shows very interesting things:
1. ch1 = 100% ail
    ch2 = 100% elevator
    ch3 = 100% Thr
    ch4 = 100% rud

2.  CH5 is used for mode select, but in combination if 2 physical switch and in fact can work with 6 modes, although Iris use these 2 switches to form 4 modes.
3. on mission planner it shows that Iris uses ch7 for RTL, but in th9x, it noted elev and !elev...since the switch is left top is defined as elev switch

setting up trainer is the go in to radio menu, (long press on + ), and left and right to find the page.
there is only 4 source can be selected.
the set up of these 4 functions are easy to understand, but there is a very strange way is +=, which is a sum up of trainer and student. := is a replacement , this is the one to use.

After all these set up, you still need to go to custom function set up for setting up a trainer function

First, start with instructor's radio, power it on, plug in the cord, and plug another cord into student, student's radio will start up with out power on the switch.
Then on the trainner's radio, go to the trainner set up, and do the cali.