Barbie is everything. He’s just … Robert Oppenheimer.

That’s right. The main character competing with Barbie for attention right now isn’t Ken, her plastic significant other. It’s the man who designed the atomic bomb.

Fans have been waiting for this summer’s release of two movies — “Barbie,” from Warner Bros. and directed by Greta Gerwig, and “Oppenheimer,” from Universal Pictures and directed by Christopher Nolan — which are both coming out on July 21, and they have been poking fun at the stark contrast in the movies’ themes, moods and color schemes.

The result of the release schedule is a mash-up many people may not have seen coming: Barbenheimer. Or Boppenheimer, if you will.

“Oppenheimer” is Nolan’s prestige movie based on “American Prometheus,” a biography of Oppenheimer, the scientist who led the Manhattan Project, which during World War II produced the first atomic bombs. The trailers for that film, with intense music and suspenseful scenes starring a pensive-looking Cillian Murphy as Oppenheimer, are in stark contrast with the pink and sparkly trailers for “Barbie,” which show Margot Robbie as the doll living in Barbieland before setting off on an adventure into the real world.

The two characters could hardly be more different (does this Venn diagram even have a middle?). And yet, Robbie and Murphy are appearing on T-shirts and sweaters together.

Memes, videos and online chatter have flooded social media, and some people are making plans to see the two movies on the same day. A debate about which order to see them in — “Barbie” first to start the day off light, or “Oppenheimer” first, to end on a more cheerful note — hasn’t been settled.

The curious crossover is also giving rise to real-life merchandise. A Google search for “Barbenheimer T-shirt” brings tens of thousands of results, and sellers on Etsy have designed their own versions. Some feature Robbie and Murphy, while others combine Barbie’s pink font with a pink drawing of an atomic cloud.

One such T-shirt, and an early entry in the crowded field, is a simple split-screen combination of the two movie logos, spelling out “Barbenheimer” with the release date of the films.

Hunter Hudson, 23, a filmmaker in San Antonio, said he originally designed and created the shirts for him and his friends to “roll up to the Barbenheimer double feature” on July 21. But when he posted pictures of the shirt on his Twitter feed, he said, it took off beyond his expectations.

“I normally get about three or four likes on anything I post,” Hudson said. But after sharing a few mock-ups of the shirt, he woke up one morning to hundreds of messages from people asking him if they could buy it.

Hudson makes the shirts himself, with a friend, and charges $40. So far he said he had made about 150 shirts, with a second batch of about 70 more on the way. It takes him about 45 minutes to an hour to make one T-shirt, which he does by cutting two shirts in half, pinning them together and sewing and pressing them.

“I had a couple of movie theaters reach out to me privately to do bulk orders for employees,” he said. “It’s been overwhelmingly positive.”

This kind of organic marketing is probably good for both films, said Robert Mitchell, the director of theatrical insights at Gower Street, a company that does predictive analysis for the film industry.

Not that the studios’ marketing has been lacking: There are life-size cardboard Barbie boxes in theaters for people to take pictures and a selfie generator. There have been collaborations with multiple brands: The frozen yogurt chain Pinkberry is offering a Barbie flavor, Gap has a line of Barbie-themed clothes, and Airbnb is offering a real-life Barbie Dream House in Malibu. Warner Bros. declined to comment on the movie’s marketing efforts.

What all this hype means for box office results for either film is unclear, and awareness doesn’t always translate into attendance, Mitchell said. Predictions for opening weekends are tricky and a lot can still happen before July 21, said David Gross, a movie consultant who publishes a newsletter on box office numbers. Some conservative industry estimates, he said, have “Barbie” opening between $55 million and $65 million in the United States and Canada, and “Oppenheimer” between $40 million and $50 million. Both of those estimates would be strong for a fantasy comedy and a historical drama, neither of which are sequels. Superhero, big action and big animation movies usually open higher, Mr. Gross said.

Still, the hype around the films could be beneficial to the numbers. “Every time ‘Barbie’ released a trailer, ‘Oppenheimer’ would start trending,” Mitchell said.

“They’re so vastly different,” he said, “that they allow for the narrative that popped up organically: This would be strangest double bill ever.” That online conversation, he said, “is pretty much a gift for distributors.”

While social media is full of people showing off their tickets to see the double feature, it’s unclear how many really will. “But it shouldn’t matter,” Gross said. “Audiences are going to find them, and both films are going to do extremely well.”