How can I make DC-DC Communication overcome Firewall Protection?

Our organization desires to combine our active directory forest islands into one active directory forest with a root domain. The root domains will be maximum two. Apart from two or three forests that are used in offsite replication for DR purpose, the forests are separated by firewalls and do not communicate.

The cause of our problem is that our current setup is such that a lower security area cannot start communication with a higher security one. A two way communication is only possible when initiated by a DC in high security area. Is there a way of providing security that allows DC to DC communication two break through firewall? Help us.