Just one. My sister gets the other usually, and one's enough. It's the same game with minor differences- why get two? I understand the version exclusives and legendaries thing, but I never felt the need.
I got both PMD Time and Darkness, but that was an error on my parents' parts. Not blaming them though- it was a happy accident I guess. I was happy with both games. Gives me a second chance to do well if the first bombs.