Is there a switch or relay that stays on while two different 48v power sources are present and shuts off if one of the power sources goes out ?

 

Use two relays, each powered by one 48V power supply. Wire the switched lead through the two relays NO contacts in series. This creates an AND gate that requires both power supplies to be active.

 

There are power source monitoring circuits around, and some of them are rather simple. What is needed is an "AND" function. That could be implemented with a single IC, a dual solid-state relay.