In Sametime 8.5 you can share your screen with other users in a meeting room even if you do not have a direct network connection. It is sufficient that every participant can reach the meeting server.
However if someone wants to take over control of another user's screen he must be able to establish a direct peer-to-peer connection to the user who shares the screen.. Otherwise the user does not get a "Take control" button.
This might e.g. be the case if there is a firewall between both workstations and the port 22222 is not open between both partners.
You can check if there is a peer to peer connection between the the user who wants to take over control and the sharing user with the following command on a Windows command line:
netstat -an | find "22222"
If there is no connection on that port found, then there is no peer to peer connection established and that is the reason why taking over control is not possible.