I'm pretty sure that with Futaba both radios will need to have the square micro plugs.

From the 6EX manual:
If connecting the 6EX-2.4GHz to another 6EX-2.4GHz with the small, square “micro” trainer jack, use the “Micro to Micro”
(MM-TC) trainer cord (FUTM4415). Never connect the T6EX-2.4GHz trainer system with Futaba radios that have the
large round "DIN" connector type as it will cause your T6EX-2.4GHz to malfunction. The T6EX-2.4GHz transmitter may be
connected to another T6EX-2.4GHz, or any T4EX, T6EXA, T7C, T9C, T12Z, T14MZ, FX-40 series transmitter.

From the 7C manual:
Trainer cord - the optional training cord may be used to help a beginning pilot learn to fly easily by placing the instructor on
a separate transmitter. Note that the 7C transmitter may be connected to another 7C system, as well as to many other models
of Futaba transmitters. The 7C transmitter uses the newer rectangular type cord plug. Both new-to-new and new-to-round plug
style trainer cords are available.

To me that reads like BOTH radios can connect only to square connectors but be careful as both kinds (sq to DIN) are available. Avoid DIN radios.