You don't need software, you need to learn how to use DOS, or better yet, buy a cheap internet router! I have a similar setup and learned myself the trial by fire way. I have a pair of T-1's coming in through a Cisco 2600 router that is on one network IP address and a DSL modem on a separate IP address. I have the T-1's going to an extranet called TNS and from there I have redundant LVC's into every broker I do business with. I only use the DSL for internet and email. I started out with two network cards one for the internet and one for the T-1's. This is messy, especially if you have multiple machines that have to access both services.
It is not the way to go, but YOU CAN USE WHAT YOU HAVE NOW if you go to DOS and learn how to use the route command. Just type route in a DOS window and all of the functionality is explained briefly. Basically you can add as many routes as you like that are defined by IP address. For example, if you want all the traffic going to 172.80.97.* ,where * is any number between 0 and 255, the route command can direct that traffic to the appropriate card. The whole purpose of the route command is to do exactly what you described.
An easier way to solve your problem (and what I finally did myself) is to get an internet router. You can just get a cheapo 3com or a Linksys internet router (I got the Linksys). Now you can simplify things by throwing away your 2nd NIC card in all your machines, and clearing out all the routes you had to add. The trick is configuring the router yourself. No one on God's green earth knows how to do it correctly and none of the routers made for home use even support the routing function (sad but true). So you can either pay some networking company 4k for another Cisco router and 2K to configure it or you can do it yourself like I did. There is really no need for Cisco stability though, if you are using the internet just for email and research. If my linksys were to burn while I'm trading then I only lose the internet and email. The connection to my brokers would remain.
To configure your new router, find out what your network IP of your brokers is, and change the IP of the router to that network. Then set the router to be a dhcp server and give out IP's on that same network. Your pc's NIC will simply be set to find dhcp. Plug your T-1 ethernet into the wan port on the router and configure the router.
Note - the router's IP can be set to your brokers network while the wan port is configured to work with the network from your t-1.
Next, add fixed routes to your router that tell it to look for your broker on the routers switch.
Plug all the PC's and the broker ethernet into your routers switch, or better yet, add a cable from your routers ethernet into your current switch containing all your pc's and brokers ethernet.
Good luck,
It sucks.