If those two domains are children of a forest, all you need to do is use port 3268 (instead of the standard LDAP port) against your top level doman controllers. You will have problems if you have duplicate usernames between your child domains, but if you do not this works well.
Your base DN will be the top level domain. In you example, the base DN might be dc=entry,dc=example,dc=com if entry.example.com is the top level domain.
Hope this helps…
-Andy