
maidenholmes:
This really needs an explanation from the topic poster. My first thought was of two people who know each other very well, either actually or instinctively, and who work/live together without needing to constantly adjust.
Not the topic poster, but similarly to what AthenaTheStorierX said, it means a couple that gives you the same impression as an older, married couple. They bicker and tease over small things, but do it since they know each other so well and are used to always being together.