No, you cannot have 2 different cable modems hooked up on 1 line. There's no way the cable company would allow that.
Your cable service provider will more than likely MAC filter their cable modems, and I believe most cable modems TFTP their config upon boot. If you got a new cable modem, you could probably call your provider and get the MAC added (assuming you need to), but I'd personally be nervous hooking more than one up. No reason to get yourself cancelled for something like that.
Do you really need the extra public IP's? In my experience, cable company's don't route blocks to residential customers. You're better off making whatever you need work with NAT and port forwarding, or switching to a provider that's willing to route you a /30 or something (though you will most likely pay a premium for the extra address(es)).
