I'm not sure, but if you subscribe to two cable internet companies if it is possible then you probably can.
A wireless router might be a lot cheaper for sharing an internet connection. All you need are two very long network cables, or a wireless card if one computer is very far away.
The Modem will only have one ethernet jack for you to plug in to your computer, but you can get a wired or a wireless router so you can connect more devices to the new work.
I believe you are getting confused with the MAC address of a PC.
The only possible way for you to obtain a different IP address would be to have two internet providers. They would each need their own dedicated phone line to give off a different IP address.
EDIT: I would also advise you to gain a wireless router for multiple PC's as you would need a Ethernet cable going from each PC to the router (messy job). If you contact your ISP there is a chance they will provide you with a free wireless router.
As I know a few who work for the cable companies.....
Comcast, USA
Time Warner, USA
Charter, USA
Cox, USA
You can only have 1 Modem per account (for residential accounts). So unless you want to pay for 2 accounts, you're out of luck. You're best bet would be to buy yourself a wireless N router. You could either use the wireless or connect it directly.
I use Cat7 wires here, rather then the Cat5 wires. I know a place which sells them cheaper and for the added boost... It was worth the wait for shipping.