control 3 status LEDs with 2 PCs
As mentioned, this is a simple diode OR bridge. It allows two sources to power a single circuit without back feeding each other. Prevents driving an output high when its own supply is off. Keep in mind that whichever has the higher voltage will be the actual source, but you already started that this is not an issue.

As you know, there must be a common ground between the two devices for this to work.