Play the same sound in both of your AirPods
When you use supported AirPods with iPhone, iPad, Mac, or Apple Watch, you can use Mono Audio, rather than stereo sound, to play the same content in both ears.
iPhone or iPad: Go to the Settings app
, then tap Accessibility. Tap Audio & Visual, then turn on Mono Audio.Apple Watch: Go to the Settings app
, tap Accessibility, then turn on Mono Audio below Hearing.Mac: Go to the System Settings app
, then click Accessibility 
in the sidebar. Click Audio, then turn on “Play stereo audio as mono.”
Note: When you use Mono Audio, you can’t use Spatial Audio and head tracking.