Good one, Jillybean!
I have weak knees - come from a long line of bad-kneed folks. I only use one riser when doing step - I still get a great workout - the intensity can always be upped by making movements larger and/or more pronounced.
The thing that has made a HUGE difference in reducing my knee pain is lunging onto the step with my front leg. I never do lunges on the floor anymore. Using the step still works the target muscles, and puts slightly more emphasis on the glutes, which I appreciate.