Normal rules of the road for 2G4 radios.The receivers bind to the transmitter which has its unique code.
Any number of receivers can bind to one transmitter by being "taught" that code. In most cases, once bound, if all are switched on, they will all respond together.
To bind a receiver to a transmitter, physical access to the receiver is needed.
This assumes that the receivers used and the transmitter are compatible. True for most brands, not just Futaba. With some brands, compatibility might not exist on the same model number.