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.
There is way to exchange messages between Skype & Teams.
First you need put all the user who are using Skype for business client to SfBWithTeamsCollabAndMeetings
Than, put all the user who are using Teams client to UpgradeToTeams.
Wait for 1 or 2 hours & then re-launch the client, it should work then, users should be able to chat vice-versa.