Looks like it is a simple set up , two button remote .
"A" sends a signal to activate one relay and "B" send a signal to activate the other ?
The relays are probably a normally open circuit and close when a signal is received . You would be able to add switches to work without removing the radio receiver in case the remote should become lost or malfunction .
You can add switches to the side of the box and add a second plug in cable also . the system will not care how many normally open switches you add . Only one needs to close to operate .
A spring loaded On/Off/On rocker switch would be prefered so the A & B circuits could not be closed at the same time and open the circuit when pressure was released .