You probably can have 2 modems; you can check with Comcast at (866) 447-7333. You would have to use a splitter adapter on the line coming into your place. At one time, you could lease 2 lines coming in (for extra $$ of course); I don't know if that is still the policy. And you could either lease a modem from Comcast, or buy your own. A better solution would be a router, to which your cable modem would be connected; it is about the same cost as a modem, and it is a neater solution. And some routers have a built-in firewall, which is always a good thing to have for an "always-on" connection.
If the machines are close to each other, you can plug both of them into one modem, if the modem has two different output ports. I do it this way, and each machine can surf independant of the other at the same time.
That depends on the modem and your market area. But in any case you're making an illegal connection. To have a second PC access the network at the same time as the first, you're hijacking an IP address that doesn't belong to you. If you don't want to use a router, setup ICS on one machine and plug the second one into it. Or purchase a second IP.
why would someone need two connections? HMMMMMMMMMMMMM since seems to be a bunch of brainless people in here, if you have roommates that like to download, having ur own connections can help out loads, as they don't drag down ur internet from their downloading. kkthxbye