Running two LinuxCNC instances in parallel

This was mentioned before, and seems some work around were mentioned on github
github.com/LinuxCNC/linuxcnc/issues/2716
And here
forum.linuxcnc.org/38-general-linuxcnc-q...grams-simultaneously

Nope, not the same thing, link is to running two programs at the same time, the video is using two Mesa cards that can easily be done by LinuxCNC, as well as any combination of multiple parallel ports, Mesa, EtherCAT.
Would your situation be possible using two PC's and many IO to communicate between them with feed holds and waits and whatever necessary?

OK, that is not what i was thinking/answering, but the replies are still valid, namely most of the pin names would be the same on both instances.
But, having two separate machines/configs with two separate controllers/electronics is for sure possible ... as long as you do not need both running at the same time.

Not that i am aware of, and now we know what you're after we can easily point you in the right direction: Get another PC!
Having two machines controlled by the same PC is not safe as any issue with the PC would stop both machines, together with the materials scraped that may or may not be recoverable.
If space is the issue, most monitors have at least two inputs, some can show both inputs at the same time, some also have KVM so connecting mouse/keyboard to monitor will switch those whenever you switch monitor inputs.
Keep in mind having two PC's on the same monitor without a KVM has the inherit drawback of clicking on keyboard/mouse several times till it dawns it's the wrong PC on. Done this a lot as i have a monitor in the shop with 7 inputs and usually 2-5 PC's connected.
What about using laptops? There are several i tested that work OK for normal "non mission critical" use:
forum.linuxcnc.org/18-computer/54369-use...mesa-ethernet-boards