Power single device from two circuits
A cheap and easy solution you can try using diodes as you mentioned would be as the following schematic.

Its not the most efficient option but it would work if your fan and pump are not very high current (say less than ~3A).

Note that you have to take the voltage drop into account (somewhere close to 0.5V~0.7V depending on the diode you choose and load current), so your pump and fan would have to reliably turn on at your power supplies actual voltage minus ~0.7V.







simulate this circuit &ndash; Schematic created using CircuitLab