By using Ctrl-Shift-6 followed by the x key this will only suspend the current telnet session.
It does not disconnect from the router
By using show sessions you will see a session in place.
When hitting the enter key you are resuming your session, instead type exit while in a telnet session, this will terminate the telnet session.
You can also you the disconnect keyword while on the first router to disconnect all current sessions.
Hope this helps.