For the sake of simplicity I'm going to refer to the client taking over as client A and the one being taken over as client B.
Now. Before you give client A control, you need to assign client B something to control. Otherwise their screen will freak out and they'll probably leave the server. I suggest giving client B control of their camera object(%client.camera) and setting it to observer mode on their body. You can now give client A control of client B's body. Don't forget to change both client's %client.player variable. Otherwise when they talk, play an emote, etc. it'll be their previous player object that does it. You might also need to tell client A what client B's inventory is.