From what you've said, it sounds like they're of equal status and you've given no indication that the work you may want to do is likely to be of more obvious relevance to one than the other.
As long as this is the case I would suggest contacting them both in an initial email. If you're looking to arrange a meeting with them then you could mention that you're happy to meet with them individually or as a pair.
Contacting them both at once gives you the benefit of increasing the chance that you'll get a response, while it provides them the opportunity to decide for themselves which of them (if not both) maintains contact with you. Furthermore, even if you only get into a dialogue with one of them, because you've copied in both from the beginning the other should be able to track what's been discussed.
If my initial assumption is incorrect and it's obvious that one of them may be more relevant to your interests (e.g., if one is the "theory-person" and the other is the "methodology-person") then you may want to consider contacting that one individually, but even still I would suggest there are advantages to approaching both at once.
