Multiple PSU in a single PC
If you need to activate multiple PC power supplies from a single switch, you can use a relay to trip the power signal on a remote supply. Just connect your relay coil to a suitable power connector on the original supply, and connect the green/black wires to the switch side of the relay.
When the first power supply is activated, it will power the coil of the relay, and switch the power connection on the second supply. This is a useful trick, but you must be careful to keep the two supplies isolated. Crossing the power connections from one supply to the other would be bad. Think Ghostbusters “Don’t cross the streams” sort of bad.