how can i use my cable internet in 2 desktop system?

[BigE]

Most allow only 1 ip. A simple hub or switch will not be enough, you'll find that one will work and tihe other will not.

A router (like a wifi router) uses the single IP from the cable end and assigns private ips on the switch/wifi end. When traffic is generated output, the router changes the ip so it looks like it comes from the router through network address translation or NAT. That way, you can have multiple devices that look like one IP.