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 commentVictoria commented
Please fix: This user is currently using Skype for Business. Some Microsoft Teams features, including chat history won’t be available. I had the same problem. Had to set up a different channel to chat. It would never let me get through to that person again! Keep getting the above error message.