How do I play audio on two sets of Bluetooth headphones simultaneously on iPad Pro 2024?

I just purchased a 2024 iPad pro 13-inch (M4). I'd like to use it to watch movies while flying with my husband. I just tried to do it as a test. In bluetooth settings it said that both my airpods and his airpod max (headphones) were connected but when I started a movie the audio only came out of one of the bluetooth headphones. How do I make the audio work on both at the same time?

Thanks

[Re-Titled by Moderator]