It looks like some people in the thread below have figured it out — we actually have this. You can start a chat both ways; the system will automatically recognize if this person is on Skype for Business or on Teams.
Thanks and let me know if you have any questions.
SuphatraCraig Thoendel commented
We're not going to allow anyone to use Teams until this is fixed.
We’re currently working on defining a compact mode experience. I will post more details when we have more to share.
We will not be doing this request at this time, due to other high priority engineering that the product needs. But we really appreciate the feedback and are listening to every one that you submit in this forum. Thank you!