Goldfish and koi can live a long time. Esp. koi. Don't ask me in years, though. I'm sure someone here is more knoweldgeable.
I don't know about that particular behavior, but I do know that fish do do odd things. Many of mine love to get right under the waterfall and point their heads / mouths up into the falling stream. It's common, and the guy at the pond store said they are "playing."
But your fish's behavior doesn't seem like play (perhaps the water is cooler there, or it has movement that gives a gentle "massage"??), and it's rather odd that the long-time couple shares in the activity. I know fish have some social interaction, and, after 10 years together, surely your fish have some kind of relationship or bond.
Hopefully some phish psycologist / socologist will give a more full answer.