Twin (or more) Nic Theory - newbie

[Benjamin_Greenaway]

Hi all, as a newbie I see this problem arriving on my desk sometime soon and hoped I could use this forum to find an answer, but as yet I do not see one.
Hopefully this will lead to a tutorial or walk-thru which I will gladly write once our new servers are running.

Here's the spec of the new system we're building :- CentOS 5.2 Server, Apache 2, MySQL 5, PHP 5, (phpMyAdmin) on new Dell 1950 III Poweredge with Heartbeat failover to Dell Poweredge R200 running identical OS and services.
The plan is to build a failover config where server two is one production day behind server one - at midnight server two asks server one for it's public_html directory and it's MySQL dump and reconfigures for failover.

For that purpose two nics are installed to the servers which are both in the same rack. They offer one nic to Heatbeat and consequently www, but nic two I would like to use for the faster connection between them on the local network for the backup.

Cent found both nics and has them correctly configured (well done Cent!) but how does my backup system know to use eth1 not eth0 for the backup transaction? Somehow I want it to know to use eth1, and I guess my lack of theory is confusing me the more I think about this. Please Help!