multiple power source and diode protection
If you are concerned that any volt drop might be too much, consider using a dual supply hot-swap controller that has wire-OR functionality: -



The LT4236 arbitrates between one supply or the other and selects the appropriate MOSFET for the supply that is present or has the higher incoming voltage. As both are 12 volts it doesn't matter; one supply will be selected and the volt drop of an SiR158DP will be less than 10 mV for 12 volt incoming.

Linear Technology have a range of these devices.