First of all, I have poor experience on IIS, I'm more a linux/apache/nginx guy.
Now I have a proprietary app running on an old IIS (w2003, I know is out of support) which is currently authenticating against domainA using integrated windows authentication.
I need to make it authenticate against a completely separate w2003 AD, too. I can route between the two networks. This works all on internal networks.
The app currently maps each domainA user to its personal account & details, the same should happen for domainB users. Obviously I can't change the app in any way.
I guess I could set a trust between domainA and domainB, but if possible it would be best to avoid this. Long story.
I wonder if there could be other methods, eg: make IIS authenticate users also against a LDAP, and make domainB AD act as that LDAP. Or else. Anything that works could be interesting.