An alternative to this "problem" would be the use of two mods. I recommend that you use:
RTS camera: well known mod which one of its many features allows you to switch sides (control agent) ingame with a button in a very friendly UI.
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Enhanced Battle Test: A much more advanced custom battle format than the native one (which should be made equal, imho) where you can select any unit from the different factions and distribute them in several groups.
I recommend this combination because I use it to test things in the game, very practical. It will be good for you because you will be able to do what you are asking for. After installing the two mods (easy), in the main menu you will have a shortcut to the
Enhanced Battle Test section above the custom battle access; click on it, in there you will be able to select any attacking or defending unit/group. You choose the side you want and apply for example 5 infantry units to the opposing side and start the battle.
Just as the battle starts, press L and you will open the
RTS camera UI in which you will see the menu to switch teams; click on it. Now you control the enemy and you can order them to stay at a point as far away from each other as possible. Press L again and then press the team switch button again.
Now you can carry out your manoeuvres without being disturbed by the enemy. I hope you find it as useful as I do.