I like the idea of either talking to them together or writing a letter to BOTH of them.
I agree that I would be rather upset if someone approached my husband before approaching me, unless it was a random romantic gesture about figuring out what I like with his help. Asking his permission? Not cool.
I also understand that talking to the wife first might seem like going behind the friend's back, but I would think as long as it was brought to his attention within a day of the discussion happening (or even later on in the first conversation), it wouldn't be too devastating. This would depend on him and his personality, though.
I think the safest bet is to talk to or write to both of them together so that neither one feels slighted. The only problem with this is that if she doesn't return your feelings you have to do deal with the rejection in front of your best friend.
Sticky situation. I hope whatever you decide to do works out well!