Power 12v Pump with two PSUs that may or may not both be on
You only need two diodes.

You can get dual diodes in a single package, and perhaps consider getting Schottky diodes with very low forward voltage drop. 

Depending on the current drawn by the pump perhaps something like the MBR4060 dual diode in a TO220 style case.







simulate this circuit &ndash; Schematic created using CircuitLab