Hello Yanno,
Thanks for reaching out to the Apple Support Communities! We understand you are experiencing an issue with setting up your HomePods as a stereo pair. We are happy to help with this issue.
To start, we want to make sure that the HomePods are being set up properly as a stereo pair. Take a look at the steps listed in the HomePod user guide that goes over how to set up the HomePods for stereo use:
Set up HomePod stereo pairing
If the issue persists, there are a few more things we would like to go over:
We want to make sure that the source device being linked to your HomePods is up to date on its software version. So as an example, if you are using your iPhone as the source device, we want to make sure that it has the most current iOS version by going to Settings > General > Software Update.
Make sure that the HomePods have unique names and aren't named the same. For example, make sure both are not named "Living room HomePod".
Try swapping the left and right channels of the stereo pair once they are set up.
Try restarting the source device and the HomePods.
Try assigning the HomePods to a new room in the Home app and setting them up as a new pair.
If you continue to experience these issues after going over the above steps, please reach out to Apple Support directly so we can investigate the issue further: Get Support
We hope this information helps.
Best Regards.