FS-T6 has a build in connector for another radio to be used as a trainer. However, the connector is rather unusual S-VIDEO one. There is a discussion about making a cable in the rcgroups:
The connector is on the back of the radio. It has 4 active pins and it uses connector shield for ground. Using standard s-video (see above wikipedia link) pins the signals on the radio are:
1 - PPM TX
2 - PPM RX
3 - SERIAL TX
4 - SERIAL RX
connector ring - GND
The signals that needed to be connected between the two T6s are #1 and #2 crossed (ie TX of one radio goes to RX of another and vice-versa). I have decided to cross both PPM TX/RX and Serial TX/RX to avoid output shorts...(ie output driver another output). Maybe one day we find a use for two radios communicating through a serial link? This essentially creates a "null modem" cable.
To make a cable you have 2 choices: 1) buy 2 plain connectors and solder the wires 2) rewire a standard s-video cable. Being a frugal guy I have opted for #2 (s-video cables are available for <$4 while single s-video connectors are more). However, standard S-VIDEO cables do not connect the grounds and connections are straight. Here, to connect two T6s you need to "cross the streams" and add the ground wire.
Start with taking out the outside rubber. I cut the connectors first, Start cutting from the cable side to the connector to avoid cable damage (if you slip the knife). Then pull the rubber off.
After cutting, unwrapping the shields, stripping the middle wire and re-soldering crossed over, I have wrapped them in electrical insulating tape. Note, the cross over is 1<-->2 and 3<-->4 which are on different cables (connect shield of one cable to center lead of another for each cable.).-->-->
Than next I have added the ground wire. I have soldered it directly to the outside metal ring on both ends, then slipped the wire into a groove cut out in the connector plastic.
The finished cable looks rahter nagly (nasty&ugly) but it works. However, the s-video connectors do not make solid mechanical plug. So if your plugs fall out as much as mine you'd want to lead the cable through the radio handles first and then plug it in.
My son and I flew together last week! It works!