Hi Jamie,
It is expected that you'll enquire about several programmes, even at the same uni, because it's just commonsense that you need to find the best programme for you. This should involve talking to all of the supervisors of the respective programmes, and don't be concerned about mentioning that you're also talking to their colleagues. At the same uni they won't worry about that. When it comes to applying, though, the idea is that from your enquiries you have deduced the best programme and will only apply to that. I was always told that it doesn't look good applying to two PhDs at the same university. You need to show that you've committed to one or the other and stick to that - it's all part of showing that you're ready for the PhD.
Hope that helps!