Additional file-related features and issues within Microsoft Teams for business
Dear AdamCNTW,
Thanks for posting in the community.
For your concern, it is available to meet your requirement. According to your description, I assume you want the two teams can co-author the file, but you don’t want one of the teams to have permission to access the location where the file is stored, like the SharePoint library or the SharePoint site, just these files.
If so, no need to create 2 different files for the two teams, you can go to the SharePoint site, go to the library, create Folder A for these files, and move these files to Folder A. Select Folder A, click Share, choose Specific people, then share Folder A to another team. After that, that team can only access Folder A and edit files in Folder A. All users will see changes from two teams.
We look forward to your response. Thanks for your cooperation.
Sincerely,
George | Microsoft Community Moderator