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Dear Micheal Mallo,
Good day!!
I understand your requirement and would like to confirm whether you want the external users need to access the content in your SharePoint Online with secondary authentication apart from their login details.
If yes, to my knowledge you requirement can’t achieve via SharePoint Online but you can try to check with the Azure experts to see whether it can be achievable to setup the custom authentication for the external users of the organization.
Here is the reference community for the getting in touch with the Azure experts: Azure Community Support | Microsoft Azure
Appreciate your patience and understanding. Have a good day!!
Best Regards,
Sophia