How can one let two users share the same home directory in Xubuntu 20.04

I would not recommend using the same home directory for multiple users, because important config files are located in the home directory. These are read and written by various programs which may cause conflicts between users. Plus some of these files need to be accessible only by one user, otherwise the related program will refuse to work. It's better to create a subdirectory within one user's home directory that will be used by both users, and symlink that subdirectory in other user's home directory. Of course the subdirectory needs to be group writable by group admx and have the setgid bit set, so both users can create files within this directory and they will be automatically assigned to the group.

For convenience, you may put a cd command that changes to this subdirectory in each user's .profile file, so after login both users will start in that subdirectory and not in their home directory.

Despite the warnings in the first solution I have played a bit with symlink after I read this keyword.

One might do it like this:

Suppose both administrators adm1 and adm2 are in the same group admx. Suppose /home/adm1 is the home directory of adm1 and it is properly configured. No work has to be done to provide adm2 with reasonable settings, since his directory will be replaced by a softlink to /home/adm1:

sudo rm -rf /home/adm2                 # remove the original home directory of adm2.
                                       # Note: adm2 must exist before his home directory
                                       # is replaced by a soft link, otherwise owner and 
                                       # group of the pointed to directory would change!
sudo chgrp admx /home                  # intermediate: now /home belongs root:admx
sudo chmod g+ws /home                  # intermediate: set w and SGID-bit to inherit group
sudo ln -s /home/adm1 /home/adm2       # create adm2's home directory as soft link
                                       # due to SGID-bit, root:admx own the link
sudo chmod g-ws /home                  # remove rights granted in step 2 and 3
sudo chgrp root /home                  # back in original state 
sudo chown -R adm1:admx /home/adm1     # make sure that adm2 gets access rights through group admx
sudo chmod -R g+s /home/adm1/          # establish group inheritance in favor of admx
sudo chmod -R g+w /home/adm1/.config/* # there might be programs like x-tile wanting to write there

The last two step are necessary because adm2 is not owner of /home/adm1.

The advantage of this solution - albeit more complicated compared to the solution above - is that it avoids duplicating ./config (not mentioned there) to the other administrator's home directory such that both enjoy the same settings. The second administrator gets all his power through the group admx. The SGID bit set at the end makes sure that subsequently created directories below /home/adm1 also inherit group admx.