[Melrna]

Short of the Hawaiian gods having fun at your expense, I would make sure they are on two separate circuits. I would pull down the circuit breaker panel to make sure none of the wires are not on the same post, crossed onto a common hot bus bar, wired in series somehow or ground wire not shorting out somehow.
I would take voltage readings across the propane circuits while someone flushes the toilet. If you lose voltage than there is a short/cross wire somewhere. The fun begins if they are. I would than start tracing out wires for both systems to see where they are crossed. I would start tracing out the propane trident gas control panel first. You will need a voltage meter to help find this. First thing I would check on the panel is the wires - corrision, tight, connections.