An OSR game called The Blackest of Deaths has a neat mechanic that would use these two dice. Like most OSR games TBoD is d20 based, roll under stat in this case. However, any time you roll the d20, you also roll 2d6, one light and one dark. The light dice represents extra benefit, and the dark die represents complications. Whichever die is higher adds that result to the pass/fail indicated by the d20 (nothing added on a tie). This gives you a much wider range of results than you normally get in d20 systems. I thought it was neat.