Hi!
You're right, the left lever ist for the bow thruster, the right lever is for the stern thruster. You can use them independently, which belongs to you wanted movement, like side stepping or turning on the spot for example. As in real life, such a thruster is efficient at very slow speeds. So youl will have even no effect above 5 knots.
Since the "Vermaas" is a very huge ship, you have to wait a little, until the full thrust will move the ship.
Regards