I am not a robin or even a bird expert; this answer is solely from googling "Do robins mate for life?"
From Journey North American Robin -- Annenberg Media http://www.learner.org/jnorth/search/RobinNotes2.html
"No, robins do not mate for life. Pairs usually remain together during an entire breeding season, which can involve two or three nestings. However, in spring, sometimes a male and female who mated the previous year will both return to the same territory and end up together for another year. This happens most frequently when they were successful raising babies the previous year."
From the same source: "....as the last brood fledges and becomes independent, the male and female feel less attached to the territory. They grow restless to travel and associate with other robins."
From this and another source which I can now not bring up, I infer that you may have seen them late in their third brood, or they may both be briefly hanging around their seasonal territory until it is time to migrate again. (If you had just raised three broods, you too would need a short rest before flying several thousand miles.)
See also The Cornell Lab of Ornithology, All About Birds, The American Robin http://www.allaboutbirds.org/guide/American_Robin/lifehistory This source says that many American Robins "spend the whole winter in their breeding range. But because they spend more time roosting in trees and less time in your yard, you're much less likely to see them." It does not say whether the male and female stay together.