This is based on the single player mission where you have the jeep, the brdm and you'll be nailed by those two T55 (or whatever tanks) later. I forgot the name since I am at work now and cannot look it up.
Anyway, it seemed to me that to direct an attached unit, you have every time to open and close the map with f11. What I mean is, I can only give orders to the unit I clicked first on the map screen, no matter which one I select later in the same "map screen session". Then I have to close the map screen and to reopen it, select another unit and plot a waypoint for that one. Doing this, I was able to direct a rifleman and the brdm and my tank into different directions.
I am not sure if I uderstood that right, though, and maybe one of the buffs here could comment on it.