How to isolate several voltage sources that feed a single voltage meter
Since the switch is momentary contact, you don't have to have protection if only one switch can be pressed at any given time.

However, to guard against two or more switches closing, put a 5000 ohm, 1 watt resistor in series with each switch. 

Also, even with the resistors included, you can tell if a switch hasn't released if the voltmeter shows voltage when none of the switches should be closed. 

Unless the voltmeter is strange low input resistance, you won't even notice voltage drop across resistor.