I've heard of plenty of people applying for both prelim and cat. spots because they were still undecided, but the rule of thumb is not to apply to both prelim and cat at the same programs. I've heard from a few program directors and my dean that it makes the candidates seem flighty and unstable, and not someone they would want to take a risk on. They may think that you might switch out of their program later on after another change of heart, and their stats would look bad.
