Client Server Network W Separate Internet Connections on Same Network

Running windows 2003 Client Server Application, Cisco VN Router handling network connection from ISP - Network Connection shared through DHCP AND DNS on 2 Domain Controllers. All staff have share permission to access files on both domain controllers/ Lets call this connection ISP1.
Have a 2nd and separate Network connection from same ISP. Using the same type of Cisco Router. Lets call this ISP2. How do I allow staff internet access using ISP 2 while allowing them access to shared files on the Domain Controllers? I have two Cisco 48 port switches to work with - how must the new router connection be configured for internet connection and file sharing?