You can use a 3rd party SAML that supports 2 factor authentication. Several SAML/ADFS solutions exist.
@JacobBoyle412 - Hi Jacob - Please see the highlighted text in the attached pic (and at bottom). Do I take this to mean that 3rd party SAML with multifactor authentication will work with AGOL? But that the AGOL-based multifactor option won't work with a SAML login? The text in the attached pic reads: "Multifactor authentication is not supported for SAML logins,...". Sorry, I'm reading it both ways...Any help appreciated...
@NickHarvey: that explains the disconnect, I was speaking to ArcGIS Enterprise.
You should post this question in the ArcGIS Online channel, I was answering from an ArcGIS Enterprise perspective.
Hey Thanks Jacob, you were correct - Got it answered in a reply to the post below and at very bottom link (below). MFA can be done with AGOL using your own SAML basically.