Authenticating FogBugz users across multiple active directory domains is available as of FogBugz 8.4.

To use multiple LDAP domains, you can go to Admin -> Site Configuration -> Authentication and click on the LDAP tab. In the LDAP URL field, you can enter in multiple URLs separated by the | character and FogBugz will try to authentication against each of them.

To use the LDAP tab in site configuration to talk to an Active Directory server, use these settings. When you get it working with one URL (one AD domain), try adding the second URL as described above.