Is it possible to configure multiple identity providers for Enterprise Logins in ArcGIS Online?

Answer

Yes. Starting with the June 2018 update to ArcGIS Online and Portal for ArcGIS 10.6.1, organizations can be configured with multiple identity providers through a SAML-based federation of Identity Providers. The portal accesses the discovery service hosted by the federation, which provides a list of the identity providers and service providers participating in the federation.

Note:
A single SAML provider can be configured to use multiple authentication sources. Any limitations of authentication are dependent upon the SAML provider.