Cloning just... seems ripe for abuse to be honest, there are other methods, but I doubt they'd work either.
Things like World of Warcraft having "Cross Realm Zones" that allowed you to play on other servers with friends via group invites through the battletag / Real ID system. Problem was like most things they released it was half assed and full of bugs which plagued the game for a long time, some of the bugs I believe are still there. Also allowed PVP players to invade PVE/RP servers. Granted we wouldn't have that kind of problem, but it would be a lot of work, make crowded zones even more crowded, and probably bring more bugs than fixes.
