Hooking up 2 computers?

I have one comp upstairs and 1 computer downsatirs the one upstairs is using sbc yahoo dsl . When we got our 2nd comp we called sbc and asked them how we would hook up another comp 2 the internet they said all we had to do was use a splitter and plug the comp into a phone line but this still isnt working any ideas its saying that theres an error because we have the same ip address