Networking with 2 nic in one computer

I have 2 computers at home running Windows XP. One of the computers I want to use as the main computer that provide internet connection for the both using dsl. I currently have 2 network cards in one of the computers. The nic’s are installed correctly with no errors. When I plug the cable into one of the card I get the internet no problem. When I enable the second card, the internet stop working. When I disable the second card, the internet starts working again. What am I doing wrong? In network properties the advace section, I have it checked where it says network the internet from this connection. Help??