It does seem stupid to open both movies at the same time. Moviegoers are a shrinking demographic and the audience for seemingly different movies really does overlap a lot. So why the hell would two of the most hyped movies open at the same time? Both would benefit from spacing their release dates out.
Nah, if anything they're benefiting from the joint marketing. Lots of people who'd never be interested in Oppenheimer are booking double-features of it and Barbie just to get in on the Barbenheimer meme (and vice versa!).