I don’t have very much zdrive experience. But I would have to agree 100% with zdrive’s comments. Both angled slightly for astern movement and use both when going astern, varying power or azimuth in order to straighten out. Using only one sounds like a sure recipe to zig zag back and forth nearly hitting the sides of the slip. If it is the boat I am thinking of it is probably fixed pitch prop with variable speed electric motor drives, therefore you can dial in a real slow rpm order.