What I've done is created a script that copies my sons singleplayer database to my machine into the server directory so that any changes he has made in SP mode are still there when I get back from work to play MP with him
Then when we finish MP mode I copy the database from the server to his machine again so that the changes made in MP mode are in his SP version for the next time he plays it.
Seems to work fine
I do wish I had a teleport though lol
Paul