Authenticate from two different subnets

It is possible for a group of users using an access zone to authenticate across two different networks? We have a campus commodity network (already defined) and want to create a path through our DMZ for the same group of users.

However, when trying to authenticate we get "access denied" errors. Apparently the issue is that the users can't connect to the same access zone across the DMZ because they are already defined for the campus network.

Thanks in advance for any help!