Microsoft Teams is not support have more than one channel open in client.
If you need these functions, we recommend you vote for it in this link (https://microsoftteams.uservoice.com/forums/555103-public/suggestions/40463074-multiple-windows-for-channels). Microsoft will always focus on customer’s feedback and experience, some new features would be added to the services based on customers' feedback in the future, and your good ideas will be very helpful for them to improve the service.
Your time, understanding and cooperation will be highly appreciated!
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