You've already got the most effective solution -- creating two separate + and - habits. The - one is set to Hard; the positive one is set to Easy. The problem with making it a single habit is that the algorithm is based off of the current value, so the more red something is, the more damage it does.
You could, as an alternative, edit the habit before you click on it negative or positive, and set the value to the proper difficulty depending on what you were about to click. That would net you the benefit of having a static (non-decaying) value for the habit.