If I understand you correctly, you want to simultaneously serve two different libraries from the same PC.
You already realize you need to ensure the ports don't conflict and paths to media files match what's in the library.
There are two ways you should be able to do this:
1. Run a second copy of MC in a VM
2. Run two versions of MC (like 26 & 27)
If you have multiple instances of MC27 on the same machine (excluding a VM) they will still share some resources. But different versions of MC can coexist; this happens normally after a major version upgrade. So run your father's library under an instance of MC26, while yours is MC27.
#1 provides complete isolation, but #2 will probably suffice.
I don't run MC on linux so I haven't tested this out, but I'm not aware of any reason why it couldn't work. It can be done like this on Windows.