For this I've tried many things. Join command not good if you switch like this :
Squad 1 12 units current player squad
Squad 2 12 units
If you do this
Squad 1 join grpnull
Squad 2 join player
The AI of squad 1 is disable --- they are stuck
Only 2 methods : all by script, or swithcamera "group".
By script:
With a user marker you can define a position with this you can order to squads, units, vehicles to go to this point.
It's the best and realistic method. But you need many scripts and tasks for manage this. The tools need for this are radio, variables and arrays. With radio you define the current squad you "use", store it in variable, player put a marker, you trap the position of it and then with this you can move the squad to the position.
Switchcamera "group"
More easier, but less powerfull. A problem the radio don't work very well when you are "in" other squads, so you need action for each leader. Work like a helmet camera, but you can't give directly orders to the leader, for move it you can use a waypoint set to false and always visible, and you move directly it on the map.
As you can see it's not really easy to have multi-squad.
As Crassus says Spinor have done this. You can dowload CE1 here, or CE2 in thechainofcommand, and see the scripts.