Can two MacOSes share some users or home folders?

Solution 1:

The problem is that different OS versions may define or parse 'user configuration data' in different ways. So keys in .plist files might use different names; the range of acceptable parameters for a key might vary.

There are also other data types, such as sqlite3 databases, which might similarly have different tables, different names, and different data.

For example, 'downgrading' the data for Mail.app to an older version may not be possible, or lead to data corruption.

However, simply having a common area for documents is common practice -- as long as your OSes don't have different versions of applications that might use different data formats.