As stated in the last update, the ability to use multiple Teams accounts at the same time is being implemented in stages for Desktop. Faster tenant switching functionality on the Windows client has been rolled out to the public ring fully, and Mac will begin rolling out soon. Work is in progress on cross-tenant notification improvements, and support for multiple accounts on desktop will follow. We’ll keep you posted!
Note: The ability to use multiple Teams accounts at the same time is supported on iOS & Android today.
Alex & the Teams team
EditedChristian Haberl commented
It is a catastrophe for us, that after switching off Skype for Business, all my employees are not reacheable in our teams tenant anymore, when they are in a client's tenant. Fix this fast, or we will need another messaging solution.
This feature request is working its way through the backlog queue. No new details are available yet.