is the 2x AD on the same domain? and if not are they in two way trust?
If they are on 2 way trust, you can create a single authentication realm in WSA and put both of the AD servers.
If they are not, you can create 2x separate authentication realm (i think starting version 7.7 and above for WSA)
Then in the Identity you can enable the authentication and select the authentication realm to use.
Or configure authentication realm sequence for both authentication realm and in Identity select the authentication realm sequence profile.
