At the time of writing, out of the box, there is not a way to allow users from more than one active directory to access your ArcGIS Server.
ArcGIS Server can hook into a single active directory. I am not a security boff, but you could investigate (stack overflow question?) the possibilities combining domains from different active directories with other products or at the domain controller end. I have heard of this being done, but cannot provide you with anything concrete.
ArcGIS.com allows you to setup Enterprise Logins, which makes use of SAML Web Single Sign On. SAML is an open standard to securely exchange authentication and authorization data between an identity provider (your organization) and a service provider (in this case, ArcGIS Online). Now, I would like to be corrected on this, but as far as I can see, whilst it is possible to consume multiple identity stores with the SAML model, ArcGIS.com only appears to allow you to point to one authentication server. Therefore, only one active directory. This might be by design, as what happens when it finds 'Adam Smith' in multiple stores?
As for Portal, this also relies on there being a singular identity store (i.e. One Active Directory).