Need circuit to toggle voltage between two paths
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Relay voltage source switch.


When V2 is not preset V1 will power LOAD1.
When V2 is high enough to energise the relay the V1 will power LOAD2. 




Figure 2. A typical relay.


Choose the relay coil voltage to match V2. 
Choose relay contact voltage and current ratings to meet or exceed the supply voltage and load current. 
If using DC check that they are adequately rated for DC.