Congratulations on your two new relationships!
Sounds like you are off to a great start.
I have had some great metamour experiences and some not-so-great ones (and one downright awful). The key seems to be communication, as much as possible. Talking things out, having meetings, sharing a common email thread or FB chat, all of the above - whatever works best in each situation. I am currently in a relationship with a married man and I feel it has been really beneficial that his wife and I were friends prior, and have our own communication already worked out. It has been helpful for us to connect both separately from him, and in three-way communication when any of the three of us has questions or doubts.
One thing that I did, though it may not be for everyone, was to go through this:
It offered me a lot of perspective and also gave me a starting point for drafting a letter to them in the beginning outlining questions I had about their boundaries and preferences.
Like I said, not for everyone, but it's an option!