It seems that that app protection policy with Outlook that restrict the second user to log in. Please check if the Outlook app has met the following requirements:
• The end user must have the Outlook mobile app installed to their device.
• The end user must have an Microsoft 365 Exchange Online mailbox and license linked to their Azure Active Directory account.
https://learn.microsoft.com/en-us/mem/intune/apps/app-protection-policy#outlook-mobile-app
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