WARNING: Do not connect an older "DX6" to a DX6i, DX5e or DX7! It has twice the voltage and will do nasty things to your newer transmitters, like blow fuses!
The ParkZone Radian that me and my father just chipped in for together was an RTF kit, which included a Spektrum DX5e transmitter. I already have a DX6i, so the two of them together created a very powerful capability: the Trainer function.
All the new Spektrum 2.4Ghz gear (DX6i, DX5e and DX7) can be hooked up together to create a buddy box setup which allows one of you to be the master (active) transmitter, and the other to be the slave. Whichever plane the master transmitter is bound to can be controlled by the slave transmitter as long as the operator of the master transmitter is holding the "Trainer" spring-loaded switch.
It's very easy, and all you need is a trainer cable (and of course the two transmitters). "But wait!", I hear you say, "where do I get a trainer cable?". Well, you can buy an official one from JR or Spektrum, but nobody seems to advertise these and you can bet they will be overpriced. Save yourself the money and buy a simple mono "headphone" cable which is male at both ends, like so:
If you happen to only have a stereo cable, that will also work just as well. Alternatively, if you're a junk hoarder like me you only need to dig through all your cables until you find two cables which both have these small headphone jack ends on them, and then solder them together. The cable is "straight through", meaning that at both ends of the cable the wires connect to the same part of the headphone plug.
Spektrum recommends you don't use a cable longer than 15 feet. Personally I think the message should have been "don't use cables shorter than six feet!". If your cables are too short, one of the operators will spin around or do something else ham-fisted and end up yanking the other person's transmitter out of their hands. THIS WOULD BE BAD, especially if that was the master transmitter which was just tossed to the ground.
Anyway, once you have your cable, you just need to connect it to each transmitter (it's on the side of the DX5e, and the back of the DX6i). We're going to use the DX6i as the master here, as it has greater range of functions for servo mixing and the like.
You will notice as soon as you plug the trainer cable into either of the transmitters, they will switch themselves on. This is normal - your power switches on the front of the transmitters now define which one is the master and which is the slave, like so:
Note that the DX6i on the left has its power switch ON and the DX5e is OFF. This has set the DX6i as the master transmitter. From here on, the DX6i will operate as normal until the "trainer" switch is held. When held on, the sticks on the DX6i will not respond and all control inputs are taken from the Slave transmitter.
What's nice about this solution is that the "instructor" can hold that trainer switch until the trainee makes a dog's breakfast of flying. The instructor then can release the switch and have immediate control of the aircraft.
Now let us cover the critical things to do BEFORE YOU GO FLYING:
First, make sure all servo reversal switches on the slave match the settings on the master. If you don't do this, when the trainee takes over they will instantly turn the wrong way, dive rather than climb, and possibly go to full throttle when their stick is at idle. THIS WOULD BE BAD.
Next, TEST THE CONTROLS ON THE GROUND. Use the trainer switch to let the trainee take over and make sure their stick inputs make the control surfaces go the right way and that the throttle functions as expected.
Lastly, define that each person must set their control sticks in the failsafe position when not in control. THIS MEANS LEAVING THE THROTTLE AT IDLE! If you are the instructor and give control to the slave while leaving your throttle at 100%, then when the trainee gets ham fisted and you release the trainer switch you will accelerate into the ground at warp factor 10. THIS WOULD ALSO BE BAD. I recommend setting up some pre-agreed language for when instructing, like "instructor has control" and "slave has control", and to mentally associated those statements with the action of setting your throttle to zero when control is taken by the other person.