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.
Thanks!Gus Lopez commented
Thank goodness this is on the works because it's essential if MS is pushing for adoption of Teams. Users have to work around it use Chrome (A Google product) in order to emulate basic OOTB Windows functionality on this system.