get enemy radio.
press 'g' to open radio menu, and select enemy radio's squad channel.
double-tap capslock so it switches to enemy platoon channel.
it now also has locked your voice transmit so you are talking on their platoon channel and they can hear you

doulble-tap capslock used to keep your voice keyed, that key is not bound in the controls but it is like still activating, so double-tap capslock is switching channels and also doing the transmit.