There are likely two different PTT circuits, one that puts the headset mic audio onto the local intercom bus (or is that one on a VOX?), and the other other is for the driver or copilot only to connect their respective mic to the FM radio.
In aircraft, the local intercom has multiple mics active, any one of which can open the intercom VOX so that crew mates will hear what is being said. The VOX circuit is there so that cockpit noise is muted when no one is talking. If the pilot or copilot wants to transmit on the radio, they mash a PTT switch on the yoke (stick), which isolates just that one mic away from the intercom, and redirects it to a radio. The crew can keep blabbering even while the pilot is talking on the radio.
My question remains. Does a back-seater have to push a switch to be heard on the local intercom, or is there a VOX?