How to make one user account to be like root for some other user account, e.g. to be able to read, write, chmod all it's files, chown from this account to master and back, kill/ptrace all it's processes and to all thinks root can, but limited only to that particular slave account?

Now I'm simulating this by allowing "master" user to "sudo -u slaveuser" and setting setfacl -dRm u:masteruser:rwx ~slaveuser.

It is useful as I run most desktop programs in separate user accounts, but need to move files between them sometimes.

If it requires some simple kernel patch it is OK.