Backbone between servers

Hi

I have a network with two server one is AD global cat and the other runs
exchange. The AD one has all the users files on it.

I have setup DFS so that the files are synchronised between the servers, so
in the event of the first going down they can still get their files off the
other server. Both servers are connected to a switch by GB and the
workstations are 100MB.

Things arent running as fast as i would expect andf think that it is due to
network load... ie one write will infact need twice as much net bandwidth
because it will replicate! Both servers are more then powerfull enough.
What i thought was if i put another GB network card into both of these
servers and then put a cross over cable between them, then it will move the
replication away from the main network. So i did that and set them with a
different subnet. I then edited both of their Hosts files so that they use
the new subnet to speak to each other. The thing is i dont think this
connection is been utilised properly. I think that if i ping, or telnet to
the other server it goes accross this new backbone, i guess because it is
performing a lookup in the hosts file. But im sure DFS isnt using it and if
i transfer a large file between the servers it still goes over the main
network!

How can i make sure that as much traffic as possible between the servers
goes on this new backbone? Do i need to give the othwer network card the
same ip address as the primary and then maybe set up a route so that
traffice to the other server uses the backbone interface?

Any help would be much appreciated!

Thanks

Stuart