We would like to federate our ADFS server with 2 partner IDPs. It appears they used the same signing certificate for both test environment federation servers. ADFS 3 therefore won't let me import the 2nd claims provider.
This comment seems to indicate that ADFS 4 (Windows Server 2016) supports multiple IDPs using the same signing certificate, but I cannot find any supporting documentation that confirms this.
Can someone confirm whether this is supported or not?
Yes - it is supported for ADFS 4.0.
I have added many Azure AD providers with no problem.
Azure AD uses a common certificate.