I don't have much advice either, but it does happen in the way you describe. I have talked online to a member of a quad that started as a couple as primary partners, and they each had a separate secondary. They introduced the two secondaries--they hit it off well---and within a couple years they all four were together as a quad.
I think any advice that's good for any triad or quad would apply. Be sure everyone has a connection to everyone else and it is nourished the best it can be.