Switching between 2 power sources using just a switch
As drawn, the commmon terminal of the switch will be powered by either "Power Source 1" or "Power Source 2" -- so connect the common terminal to the positive terminal of the load, not GND.

Circuitlab doesn't have a slide switch symbol, I'm using SW1 (a SPDT switch) to represent your slide switch. The center terminal of your slide switch is the common terminal of SW1:







simulate this circuit &ndash; Schematic created using CircuitLab

The generic load is represented by resistor R1 and LED D1, and the two power sources are represented by V1 and V2. Note that both power sources share a common ground return from the load.