I run Mach3 with Warp9 Smooth Steppers, so similar to the UC100, which means I can use any PC to run it. I have a cheap used Toshiba touch screen laptop from Ebay on one machine that runs Win10 and XP on a freebie laptop on another. I think you are better off getting away from using a parallel port and using an external motion controller to future proof (at least in a limited way, nothing lasts forever) your system. If you do it before the old set-up dies you can take time to do it at a leisurely pace.
Since a lot of external motion controllers have the equivalent of 2 parallel ports (the Smooth Steppers certainly do) on them it should be possible to run two machines from one motion controller, though most likely not at the same time. This would require two Mach3 setups which can be named to match the equipment and just run one at a time. One set up would only use port 1 and the other only port 2. This means the idea of a parallel port switch is probably unnecessary. Other options are to have 2 UC100 (£92 each) controllers and a USB switch or go for the UC400ETH (£128) which has 2 ports (and a slightly higher cost) and would require 2 interface cables (IDC26-DSUB25 £5.32) to go from the headers on the controller to your machines.
The recently bought Toshiba laptop was to replace the previous freebie laptop running XP that was getting old. The internal battery died and is not replaceable so every time I switched it on I had to go through the process of setting up the hard drive then the date so that was annoying. It also had keyboard issues and I have been using it with a wireless mini-keyboard but that, whilst it is good for some things like a remote pendant, is not so good for typing in commands. It would probably be comparable to the cost of a now rare parallel port switch and extra cables.
Laptops work fine with Mach3 despite dire warnings in the Mach3 paperwork. Just set them up to run like a desktop machine, that is to not sleep or power down in any way when powered from the external supply.
Martin C