Who in their right mind would want to split a dialup connection? Unless you don't have a problem waiting 37 minutes for each page load.
Otherwise, my line of thinking goes like maybe you have the "master" computer which creates the dialup connection. That goes out the ethernet to the WAN on the router. The router then gives that to its nodes. But in order for the 2nd PC to get online, the first PC has to connect.
And I'm not sure how you can tell Windows to use your current internet connection to go out your ethernet. Perhaps regular Internet Connection Sharing would do this? You can try.