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.
SuphatraBrandon Kerns supported this idea ·Brandon Kerns commented
Just wanted to add that I am still seeing this as an issue. We have a testbed of Teams users, and when logged in to Teams, a SfB user can be located via search and it notes that "This user is currently using Skype for Business" with the icon, but when a chat is actually sent to the SfB user it reports "Failed to send. We ran into a problem sending this message." This is the O365 SfB (not on-premises).
This is important for adoption of this product vs Slack (which is already a hard sell to the Slack-invested users) and user adoption in general, as a big issue with Slack in this area is that the team users must switch between the Slack and SfB clients depending on who they are talking to.