Cisco AAA/Identity/Nac :: ACS 5.2 AD Trust To Other Domains?

We are trying to set up ACS 5.2 in our multi-forest AD environment.  As part of our evaluation we set up an Active Directory External Identity Store to a domain (a.b.edu).  It connects properly and I can see the directory groups in the that tab when we Select.   This domain (a.b.edu) has a two way trust with another domain in another forest (x.y.b.edu).  However, I do not see the groups in that domain and I cannot seem to manually add those groups using the Add on the free text Group Name.
 
The documentation is not clear on this point:  Page 8-41 and 8-42 of the "User Guide for the Cisco Secure Access Control System 5.2) says:  "The External User Groups dialog box appears displaying a list of AD groups in the domain, as well as other trusted domains in the same forest." This implies to me that it cannot cross forests even though a trust is set up.  This seems to be what is happening.