UPDATE: This feature is not coming to Teams until sometime in 2021, Microsoft now says. –Paul
Microsoft Teams will pick up the long-awaited support for multiple users on desktop starting in December, according to the Microsoft 365 Roadmap website.
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“Teams users will be able to add additional accounts, change their profile picture, and switch between accounts and [organizations] through Settings,” The Microsoft 365 Roadmap site notes. The feature is currently in development and will ship in December to Microsoft Teams users in commercial and education worldwide.
Note that this refers to the desktop versions of the Teams client, as you can already switch between accounts on mobile, including personal accounts. If you want to switch between accounts on desktop today, you need to sign-out of the app and then sign back in with the second account, which is obviously not ideal.
Note: News of this change was first reported by OnMST; I found out about it courtesy of Mary Jo Foley.