Of all the birds that visit our feeders in winter, northern cardinals seem the most likely to travel in pairs. Chickadees, goldfinches, house finches and pine siskins usually come in groups of anywhere from three to ten. Nuthatches and woodpeckers most often appear singly, though we have seen pairs occasionally. But, almost always, if you see one of the cardinals the other is close at hand.
However, I don't often see them at the same feeder at the same time, as they happened to be a couple of days ago (above). In fact, John V. Dennis observes in A Complete Guide to Bird Feeding (Alfred A. Knopf, 1994, 2002) that a male cardinal may not let his mate eat with him all winter, though he eventually relents when spring brings the breeding season:
Instead of chasing her from the feeding tray, he now begins to offer her shucked sunflower seeds and other choice tidbits. When he brings these offerings, she crouches appreciatively with her beak open and wings vibrating. Her posture and actions are exactly the same as those of young birds begging food from parents.Maybe having the hopper-type feeder with two sides, as shown in the photo, is an advantage to the female at this time of year. He may not even have known she was there.