When you add Miro as a backup to your account, you can only select on Team in the onboarding process:
If you wish to back up multiple Miro teams with the same Pro Backup account, follow these steps:
Go to Settings > Backup
In the table, click on Add extra team
This will redirect you to the onboarding process
On the 2nd step, select the other team in the drop-down and complete the process
Note: If the other team is only accessible by another Miro user, you need to invite the user instead by clicking on Send Invite.