Poly tangles can be vees or triads or quads or quints or huge sprawling networks that require roadmaps to navigate. They can have everybody living in one house, two houses, three houses, the same town, the same state, the same continent, or merely the same planet.
A quad composed of two couples who maintain separate houses is nothing unusual.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.