You'd be better to use a ground station that did everything required before sending the signal out to the Rx. If you tried to do it using a simple bus like the picture describes it is likely that your PWM out would be either unreadable, or misread as an incorrect value. You could use an Arduino to allow the Pilot, and copilot to do different task. Or use the trainer port on a radio that would support what what you want to do, but some only work for sims.
Let's say you did different modes of control based on a 4 position switch or something:
Mode 1: So for example mode 1 the teacher could take complete control by flipping a switch
Mode 2: Teacher can take complete control, or control of that channel by just moving the sticks of his transmitter
Mode 3: Take the sum of the Teacher and student's control, so if you wanted to add throttle to what the student is inputting your input would be added, or subtracted if you wished for less and inputted a negative value
Mode 4: Teacher controls the quadcopter, student controls a gimbal
All of this mixing or buffering would be handled before the signal were transmitted, though you could have two receivers and the Arduino on the quadcopter to accomplish the same thing, with perhaps reduced range due to the two transmitters.
