According to the Yerkes-Dodson Law, there is optimal point of arousal and performance. Suppose a person takes a stimulant such as Adderall. This would increase his arousal. However, it may increase his anxiety (e.g. side effects). Would combining a "downer" (such as alcohol) with an "upper" (such as Adderall) maximize productivity? This is assuming that at a baseline level, a person cannot maximize his productivity.
I think in this case, you should look at 2 graphs of optimal performance. So one inverted bell curve (Yerkes-Dodson Law)for the stimulant, and another for the 'downer'. So in this case, each has their own optimal productivity, and an interaction of the 2 isn't just a simple combination, because this would be a third graph altogether. So unless there is something you know of the exact interation effect of the particular stimulant, and particular depressant,I wouldn't risk mixing them for optimal productivity.