In the 1990s, the pod included 10 orcas and they were frequently spotted around the UK and Ireland.
Since 2016, when a female nicknamed Lulu was found dead in the Inner Hebrides, only John Cos and Aquarius have been spotted.
The brothers were last seen off the west coast of Cornwall in May 2021.
Backshall said he had spent half an hour watching them from his boat on Wednesday.
“These two utterly majestic bull male orca were cruising around, super calm,” he explained.
“There were a couple of moments when they appeared to be jumping and diving, we thought they could have been feeding on something relatively close to the surface.
“It was just the most overwhelming, emotional, beautiful experience and even more so because they are such well-known characters.”