Controlling 1 fan with 2 different computer power supplies
The small amount of reverse leakage current in the diodes is not going to be a problem for the power supplies. The loads on the power supplies will sink the leakage current to ground.

If you remain concerned you could do these things:


Remove one of the paralleled two diodes since it is not needed. This will cut the leakage in half.
You could add a load resistor on the supply side of your diodes to ground to assure that there is a path to ground for any reverse leakage when that particular supply is off.
Just equip your vent with two fans instead of the one. These would be powered separately by each computer. This could be a wise move because when both computers are on you actually have more heat generation in the closet.