Hi panagiotis194,
Thank you for using the Apple Support Communities! I understand you are trying to stream audio via AirPlay 2, to two speakers, but you are only able to select one. I'd like to help.
Take a look over AirPlay 2 requirements first, to make sure your speakers are compatible and supported: AirPlay system requirements - Apple Support. To stream audio to multiple speakers with AirPlay 2, you would need to set up the Home app and assign your speakers to a room within the Home app: How to AirPlay audio - Apple Support. Learn more about this here: Add and manage TVs and speakers in the Home app - Apple Support.
Thanks for posting your question! Regards.