So to let you know how I've seen it work:

ISP comes out, install's two separate modems, each modem is given a static IP. Each person connects to their modem and configures their router for static IP given by ISP. Simple as that. If ISP isn't putting in two separate modems, they'll put in something that can separate the IP's. You just plug from the ISP device into your WAN port, then configure router to the ISP provided static IP info.