It's actually not difficult.
Open up the mission in the mission editor again, and find Zulu - the three Abrams tanks in the south. Click on the leader to highlight their waypoints. You will see that the waypoints look something like this
move <----- synchroed with trigger B
move
move (all the eastern route)
seek and destroy
hold <--------- snychroed with trigger A
move (these three are the western route)
move
move
seek and destroy
hold
The secret is the that the triggers are type switch. (In this case they are radio triggers, but they could be activated by anything.) When a switch trigger is fired, it diverts the group to the next waypoint.
What happens here is that the tanks go to the first move waypoint and wait. If you Radio Bravo they go the eastern route, the first few waypoints. However, if you Radio Alpha they are switched into the second set of waypoints, the western route.
If they go east, they eventually get to the hold waypoint where they will stop. There is no danger of them going on to tackle the western route.
You can read more about switch triggers in the Ed Depot, or search the forum - this question (in various forms) comes up all the time.
Well done for un-pboing the mission and having a go - I wish more people would do that.