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!Kevin Garcia commented
This could also translate to having two 'tabs' open. I would like to keep the Meetings / Agenda tab open while also chatting, and even reviewing a teams Wiki file. Three tabs I would stack vertically an on a monitor rotated vertically.