Edit file /etc/sysconfig/network-scripts/route-eth0 (or whichever interface you want to route through). You can add as many routes as you would like per interface by simply incrementing the number at the end of each statement. Once saved a restart of the network services will force a read of this route file.
The syntax of the file should look like this:
ADDRESS0=x.x.x.x
GATEWAY0=x.x.x.x
NETMASK0=x.x.x.x
ADDRESS1=x.x.x.x
GATEWAY1=x.x.x.x
NETMASK1=x.x.x.x
Enjoy!