The keyboard will connect to the computer, so can’t the mouse piggy back on that wire by plugging in through the keyboard?
Edit: Turns out I just never knew they exist all over the place. Thanks! Learning a lot of cool new keyboard features out there.
During the PS-2 era Apple was using the very similar ADB plug for input devices. It was standard on Macs of the era to have ADB passthrough on the keyboards. I can’t even recall any old Macs having two ADB ports.
When they switched to USB the passthrough came with it.
I don’t recall ever seeing PS-2 passthrough, as standard.