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.
An error occurred while saving the commentMorgan Helmen commented
How does this work? I have tried to search for my own Skype username, both using just my name and my registered Skype user name but I can't find it. So how I am supposed to start a conversation?