Ok here is my scenario:

Because our 2 gaming computers are pretty noisy, we moved them to another room in a closet. There are vent holes to the outside of the building, and now that the summer comes I want to add a small computer fan (12V 0.3A) to push the warm air out. The plan is that if one of the 2 computers is turned on the fan should spin.

What I have done so far... and it works:

I extended cables from both computers from the PSU (GND and +12V) to a little box.

I added 2x 1N5059 diodes in parallel on each +12V from the PSU going to the fan.

I know now the diodes can handle up to 2A, so one of them on each +12V line should have done the job.

As I said, it works as expected and does what it should, but, and this is my actual question:

I know diodes allow a very small amount of current in the other direction.

Do I need to be concerned that one PSU could break the other one in long term use?