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.
As a company, we have just migrated to Teams and it is frustrating that I have to keep going backwards and forwards to different, teams / chats etc and I can't open multiple tabs. I want to be working on a team and be able to have a chat with someone at the same time!