What should be done to share a /home partition between two Linux installs on the same computer?
Solution 1:
You need to keep the username to UID, and groupname to GID mappings in sync between the installs.
Add the partition to fstab of both installs.
You might find that some application has configuration file somewhere under your home directory, and the updated package may make changes to the configuration that are not compatible with the previous release used on the other install. As a safety precaution it is important to keep good backups of files.