"They're just as much people watching as we are whale watching," said Steve Lake, a crew member for Iceberg/Cetacean Quest Ocean Tours in St. John's, N.L.
The two whales have been known to swim right up to the side of the boat, slapping their huge pectoral fins and waving their tails.
"People are blown away by it, absolutely," Lake said. "They can't seem to figure out how they're doing it or how we're getting them to do it. They seem to think that we're feeding them or giving them some sort of treat, but they're just doing it for their own pleasure.
"Every time one does one thing, the other one tries to up him a little bit."
Lake said the playful whales, recognizable by their tails and white markings, are never seen apart.
"We think they're sort of twins. It's very rare for humpbacks to have twins but it's possible."
Newfoundland is one of the best places in the world to see humpbacks as they arrive each summer to feast on small fish and crustaceans such as caplin, krill and mackerel.
Cetacean refers to the large marine mammals that can be seen off the island including humpbacks, massive fin whales, smaller minke whales and dolphins. Killer whales are also sometimes seen.
Capt. Barry Rogers can never promise tourists a glimpse of Mutt and Jeff because every boat trip is different as the graceful beasts cruise the waters between St. John's, Cape Spear and beyond. But he said it's an unforgettable experience for those who see them.
In 15 years of whale watching tours, he has never seen their equal.
"The most thrilling thing is when they gravitate right towards the boat," he said. "I mean, it's almost like they're people watching. They come around the boat and look at every individual and eyeball them and come a lot of times and just practically greet them. It's amazing."
Humpbacks are baleen whales with a filter-feeder system in their mouths that separates small fish and crustaceans from water as they feed. They can grow to lengths of about 18 metres and weigh up to 50 tonnes.
Sean Todd is director of Allied Whale, the College of the Atlantic's marine mammal research group in Bar Harbor, Maine.
He knows of no documented case of humpback twins to have survived in the wild. On the rare occasion of a twin pregnancy, it's unlikely both siblings would survive because of their sheer size, he said in an interview.
"They're only designed to have one calf at a time. If they were to have two calves, both calves would probably die because you've only got so much energy to divide up between the embryos."
He would like to know more about Mutt and Jeff and whether their whale relatives are in the North Atlantic Humpback catalogue, the largest of its kind, curated by Allied Whale in Bar Harbor.
The lineage of humpbacks is often traced through photographs of the underside of their distinct whale tails — the unique equivalent of a human fingerprint, Todd said.
Humpbacks don't tend to stick together for long periods except for the mother-calf bond that usually lasts through the whale's first year of life.
Todd said that the whales generally come together in pairs or groups randomly or for pragmatic reasons such as corralling schools of fish.
Humpbacks gathered in a group tend to synchronize their behaviour as they blow, dive, surface and play together.
"Why they do that, we don't know."
Mutt and Jeff have appeared off Newfoundland for two straight seasons, always together and always glad to greet visitors, Lake said.
"When one goes to one side of the boat, the other one follows.
"If you wave your arms in the air, clap your hands, yell and scream, it seems to get them a little bit more excited. They love to come up and take a look at everybody and splash people with their fins."