I have never tried this but would guess that you would need to run internet sharing on both the desk top PC and the laptop that connected by wire to the desktop. Then you would need to connect ther to laptops via a "computer-to-computer" (ad-hoc) type of network.
Why bother - buy a wireless internet gateway. attach it to your broadband modem. use a wire to Desktop and run the two laptops wireless. Then you will have WEP security and a hardware firewall in the access point/router.
Your method would leave all your computers open to hackers.