"Desk on Desk" refers to products that sit on top of an existing desk that allow one to stand at his/her workspace. The popularity of these solutions is at an all-time high due to the research on the dangers of long-term sitting, and the various ad campaigns in print and on television. While these products do facilitate the ability to stand for some users, the ergonomics of their design may create significant long-term issues for the users of such devices.
Standing for long periods of time is not healthy either. Ask anyone whose occupation involves constant standing; pharmacist, assembly line worker, physical therapist, nurse. Varicose veins, swollen legs and feet, foot pain, hip/knee/ankle pain, low back pain, are all possible risks of excessive standing.
The keys to minimizing risks in the workplace are moderation and frequent postural change. "The best posture is your next posture". We recommend an alternating postural recipe of 40 min of proper sitting, 15 minutes of standing, and 5 min of walking whenever possible. Unfortunately "desk on desk" solutions don't facilitate proper sitting since they place your keyboard, (and therefore hands) 1-2" higher above the desk; a desk height that is most likely already too high for proper ergonomics.
Take a look at the following links for greater detail on the pitfalls of "desk on desk" solutions.