Share installed libraries between Debian and Ubuntu

Or is there a better way to do it?

• Install all software you use as snaps.
• Install all software you use in a partition named /opt/. That is likely to be a source installation (tar.gz).

Those 2 methods of installations can be shared amongst distributions. The last one you will need to add something to PATH or copy a service into the main system.

You can not share /usr/ or /var/ between 2 different installations. You can share /usr/local/ though. 2 distributions can have different versions of libraries, so you may need to install a few libraries on either of those systems. But those few kb you save is not worth the time you are going to spent on this. Plus you are likely to then still break either system.

The one thing you never do is mix Debian and Ubuntu repositories, system libraries, programs, etc. It will massively break both systems. (This also extends to mixing Ubuntu and non-official variants like Mint; and while you theoretically can mix Kubuntu and Ubuntu for example of 16.04, you just shouldn't and should keep their core systems separate).

Ubuntu is on a much more rapid release cycle with much more updated software libraries than Debian has. The inverse is true as well - Debian doesn't have updated libraries, etc.

Mixing and matching will explode the systems. This is why you do not share libraries with each other.

To do so will break both systems. Do not try and share libraries and programs between the two systems.

While you technically can share user home directories across releases and distributions, you may have odd issues on one or more OSes because of differing versions of configurations by the differing versions of software on each system.

One more point I want to add to the existing answer is about apt and dpkg, and how they handle the information about installed softwares.

dpkg, which is the core package management application, uses /var/lib/dpkg/status file to store and retrieve information about debian packages installed in a system. This is very specific about your running. Even a single entry may messed the whole package management thing.

Say for example, You have version 3.18 installed for nautilus in Ubuntu. Then if you share this info, debian will see it has 3.18 for nautilus, even if it has actually 3.16. This is very simplied example. But there can be more complexity. For example, debian may see that you have packages installed from non-existing repository for no good reason and will try to remove those and vice versa.

Rinzwind's answer has illustrated one simple way to work-around with common application i.e. using self-contained applications distributed as snap or flat-pak packages. These aren't sharing libraries but if you wanted something sort of common application that may fulfill your goal.