Multi-window chat is in testing. I hope to have great news regarding rollout to the public rings soon.
We are working to ensure that multi-window and other future multi-window experiences are of the highest quality when they release.
Skype had dropped multiple chat windows last year in the move from the legacy version 7. That killed any usefulness of the new version. We just stopped using it until a multiple-window version came out early this year.
Now I'm in a new job and have to use Teams. Nice app but just one window? Arrrrrrggggggggghhhhhhhhhhhhhh! Please hurry with the new release.