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.
I have followed what Jan has recommended, however the Chat box in Teams got disappeared.
I even tried to enable SfBWithTeamsColl organization wide, and all the users Chat box got disabled as well. So I am not pretty sure that would be it.
Can someone advise on this.