I understand that you are having some issues in order to set up your Slingbox Classic on a multiple router network.
First, you need to track down the IP address provided by the first router for the Slingbox. Once you get this information, you will create a rule on the first router using the specific IP address provided for the Slingbox through the port 5001.
Then, you must use the first router WAN IP address in order to create the rule on the second router and make sure to use the same port number (5001). This way, the Slingbox will be able to reach the Internet service through a single port (5001) on both of your routers. Even though you own the Slingbox Classic, the following links will provide you with further details and information that will help you out:
The following third-party links will allow you to know how to adjust the router settings on both of your routers: