Set DNS Forwarders between domains using either one of these

Conditional Forwarder – How to configure a Conditional Forwarder in DNS

Stub Zone – How to configure a DNS Stub Zone in Windows Server

Secondary Zone – How to configure a DNS Secondary Zone in Windows Server

http://blogs.msmvps.com/acefekay/2012/09/18/what-should-i-use-a-stub-conditional-forwader-forwarder-or-secondary-zone/

http://blogs.interfacett.com/how-to-configure-forest-level-trust-in-windows-server