AmsterDan From what you describe, it sounds like you're running into limitations of the Mac file system. I'm assuming the Mac still has MacOS on it.
For comparison, I have a setup with a similar goal of opening a file on two machines and editing on either. I have a Samba share set up on my main laptop (Solus), which is shared to its VMs running various flavors of Linux. (Copy and paste doesn't work with Wayland in VMs, so that's my workaround). Here's what happens if I have a particular text file open in the editor Kate on "VM1" as well as being open in Kate on the host machine.
When I save a change to the file in VM1, I get a popup in Kate on the host machine letting me know. It allows me to choose how to handle this. I can reload the file, revert, etc. If I had the file open only in the VM, and saved a change, that change would show up when I opened it on the host machine, or vica-versa.
If you want to be able to have a file open on both computers and edit it from either, you'll need to make sure the Mac's filesystem and whatever editor you're using there will handle it. You didn't mention what software you're using on either machine, but you might look into whether they support what you want to do (like Kate does).
One suggestion, without knowing what kind of files you're working with is Libre Office which has a collaboration mode.
For larger scale situations, enterprises use a variety of software. A lot of it is aimed at businesses, like Office365. There are other web-based options used by businesses and individuals like Google Docs. There is definitely software that is designed to be collaborative by nature. A web search will turn up quite a few options.