I would probably talk to both of them. If you talk to one of them first, it might seem like you went behind the other's back, and you're friends with both.
Tell them something along the lines of "listen, I want to be clear that I value our friendship the most. But I wanted to let you know I really like [wife]. I don't know what either of you think about it or how to go from here, if anywhere. If you want to stay friends, that's fine, I don't want to lose you guys. But if you're both fine with it, I would like giving this a try".
There is no telling if the fact that you're best friends will make him feel more or less comfortable about your dating his wife. So you just need to ask. If you do it right, even if nothing happens, you should be able to stay friends. After the talk, don't expect everything to get decided right away, give them time to think about it, assume nothing is going to happen unless told otherwise and wait to see if they approach you about it.
Make sure you've cleared it with both of them before anything happens, if anything happens.