Only one user can access a FATCA account at a time. This is consistently stated across multiple sources, including the IRS FAQ and various IRS publications. Specifically, the information indicates that although login credentials can be shared among multiple designated users, only one person can be logged into the account simultaneously. If another user attempts to log in while the account is already in use, an error message will be displayed. Additionally, the system does not support multiple tabs or windows for account log-in, reinforcing the restriction to a single user at a time.
Sources:
Publication 5262 (10/2022)
Publication 5118 (07/2021)
Publication 5190 (2/2022)
