Using a DSL line as backup to one network and primary to the other

I have an Internet T1 that goes from my Cisco Router to my Netgear Firewall Router, plugged into the first WAN port. I have a DSL from another carrier plugged into the 2nd WAN port of the same Netgear Firewall Router, no problem, however, I need the DSL to also connect to some servers with public IP addresses that are provided by the carrier of the DSL as well, that will be separate from the network I have my Internet T1 plugged into. I am basically trying to use the DSL for backup on one network and primary on the other. Any suggestions?