It will depend on your assist method.

If you're using something like Keyclone.
You'll have a hotkey to switch from region 1 to region 2.
You'll probably have a warcraft macro (for every toon) on the same keybind.
So when Toon A/Region 1 is the leader, macro 1 is run.
And when Toon B/Region 2 is the leader, macro 2 is run.
Those macros will run the Jamba follow command, and set Focus or whatever you use to lead with.

If your software does that stuff for you, automatically on a switch.
Then you need to add the relevant Jamba command to the "on a switch do this" option.