I should clarify something and I apologize for not doing so earlier.
By "they should have only one personal countdown," I mean one thread, not necessarily one countdown format. Like Ryan suggested, you could have different formats located in one thread. This way it is not only organized but also allows both/all countdowns to be noticed because they are located in the same thread.
An example of this would probably be
DJ's countdown thread. He seems to have both a weekly countdown format and a daily countdown format both in the same thread.