Using VM workstation 9. I have setup 2 domain controllers each running exchange has IP and the other is running fine and the users in the respective domains can email each thru outlook.but the problem is; how do I route the 2 networks to have users email across the domains.I tried to configure 2 servers as routers but am stuck.. This my school project. Guys help:-)

You can create a router using one additional VM running windows server with two NICs. One NIC on each subnet. Enable RRAS, no special router are needed since the networks will be local to the RRAS server

Then configure the hosts on each subnet with their default gateway address which will be the interface of the RRAS server on their subnet.

Here is a video on how to set this up: Enable LAN Routing on Windows Server 2008 R2

Routing is just step 1. You do need to have the something act as the router.... plenty of ISOs for linux based systems as well if you don't want to follow JorgeM's good advice.

After which, you need to setup DNS with the proper MX records so that each exchange server for and can find the Mail handler for the other domain. emails, must be able to run a lookup for the MX of to find the mail server to service email for that address.