You could do that by having the same user id (UID) for both users, but it is a really bad practice and may have unintended consequences. See this for a comprehensive discussion about it.
A better way is either having those users in the same group and setting R or RW group permissions on their homes, or better still using extended attributes (man
setfattr), or allowing your user to
sudo su - <anotheruser> via
/etc/sudoers file, or allowing your user to sudo to root.
Same group option
First create a special group, say team_a:
Then set that group to your users (say
user2). By default each user is created in a new group with the same name as the user. You'll set team_a as the main group, and leave the individual groups as secondary groups:
for user in user1 user2; do usermod -g team_a $user; usermod -a -G $user $user; done
Then update permissions on user2's home, giving full access to any members of the
team_a group, to the folder and any files and folders contained in it, recursively; this would allow any member of
user2's home folder and files, but you're not granting access on any other users (e.g. the following doesn't provide access to members of
team_a to your
user1's home folder):
chmod -R g+rwX ~user2
user2 could remove those permissions at any time, or create new files without granting access permissions to the group.
Finally, ensure that your other user(s) will create any files with full access for members of that group:
echo "umask 002" >> ~user2/.profile
Your user belonging to other users' default group:
Perhaps simpler than the above, you can add your
user1 user to the default groups of any other users. By default in most Linux distros and UNIX systems, the default group permissions on new files is full access to the group when the user's group name is the same as the user's name, so there's no need to manipulate
umask for those users in this case.
user1 to belong also in
user2's default group:
usermod -a -G user2 user1
Granting access access to members of
user2 group to
user2's home folder:
chmod g+rwX ~user2