How can I power a load from two separate circuit breakers?
Your original circuit was very foolish: you back-fed both breakers so that someone working on what they thought was an isolated circuit (having switched off that breaker) would be exposed to live wiring if the switch on the other phase turned on.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Dual supply lamp circuit.

You can achieve isolation using one relay as shown in Figure 1. If SW2 is closed the load will be supplied by L2 regardless of SW1 status.

You need to clearly label this control box and light fitting to state that it has dual supply - ideally giving the circuit breaker location and numbers that need to be isolated to fully isolate the circuit for maintenance.