.. you really need to provide more detail!
At a guess both systems are in the same domain, i.e. server1.mydomain.com & server2.mydomain.com
Turn off DHCP on server2 (the internal server)
Remove, rename, or delete host entries on each server that are not used or relevant. Add each server to the other servers hosname list using the MAC address to lock each server to a static IP (as each relates to the other).
You could use ddclient on server2 to get the user credentials from server1 (and keep them in sync) if there are authenticated service required on each box.