You do not need to edit asound with max2play. All your Squeezelite players can be synced via web interface or app through the Squeezebox Server/LMS. You can just select which rooms should get which playback and sync or unsync them at will. Popular apps for this are iPeng on iOS or Squeezer on Android.
@alex005, thx for the link. I will check it
@Heiner, I think you missunderstood.
I have one PI, where I want to use the 2 audio outputs for different rooms.
(not multiple PIs)