Some quick thoughts: How much time? - in two of the most intriguing cities of Europe? that's tough. Prague can really only be seen on foot and then, if you wish to avoid crowds, really only between an hour before sunrise and 7:30am, so at least two or three mornings of walking up around the Cathedral. 'Pest is very different and bigger but again a couple of very early morning walks in the Castle area. My one regret and a risky (likely forbidden) but potentially rewarding recommendation for photography is the old Turkish Baths in Pest. If you can get a small camera past the steely-eyed attendants into the baths proper, you could get some amazing (very low-light) stuff (but I don't recommend the massages unless you feel a masochistic impulse to be beaten to a pulp). My recollections of visting this place are vivid and I wish I had some kind of record of it. I would walk both cities with a tripod in the very early morning and strictly hand-held through the days and the crowds. You will get some great documentary/street photography. Quite a switch from penguins!