The secret to preventing instability (motor-boating) with a high gain circuit like this is to power the speaker from a separate power supply! You can connect an extra station (or two extra stations) to this circuit design. Build the circuit into two separated boxes (for 2 stations). On each box should be use 2 ports for external power supply port
(if you use external power supply, not battery) and connection port to other stations. Create some holes for speaker push on switch.