Very basic questions

Could I then control several separate machines (each with their own breakout box) by manually switching the parallel port between the breakout boxes and selecting the appropriate LinuxCNC config?

I'm just thinking that this would enable me to control several different relatively simple machines like 3D printer, vinyl cutter (not Bridgeport or lathe) from a PC I have with an EPP capable parallel port, but no spare PCI slots.

you can do this but not at the same time
i guess you woudt like to run more then one mashine at the same PC at the same time
that is not posible at a realtime aplication

if it is a 3AXIS eatch

you can run both together using XYZ to the first and UVW to the second
using 2 parport so one for eatch mashine
they then run on the same G-code file to different direction
Wehred but posible

You may want to look at the raspberry pi4 thread. It would give you inexpensive CPUs but would require Ethernet cards. I'm not sure if they have it fully working

You could probably find workable PC's at a salvage center, pop in a cheap ssd

I was thinking of a 25 way switch, with all contacts switched. I think I have one.

I have 2 spare PCs and figuring out how I can best utilise them for different projects that I have in mind. I certainly don't intend buying another PC.

Thank you for all your help. I've got most of it pretty clear in my mind now.