You cannot connect two computers simultaneously to the Cable Modem. The ISP only allocates one IP address and this will go to the first computer to connect. The second PC won't get an IP address and, as every computer on the internet must have a unique IP address, it won't get online.
To use the two simultaneously, either you install a router, or you connect the two computers together and enable "Internet Connection Sharing" on the internet connection of the one PC that connects to the internet.
With a Router, it is the router which connects to the ISP and gets the one IP address. It then allocates IP addresses to the networked computers, using one of the IP address ranges reserved for use with Local Area Networks, and "routes" the internet traffic to the appropriate PC in the network.