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.