Honestly, i don't think that developers would tell you the way they detect cheaters..... but some thins are sensitive.
By the way, 2 people can play with different teams using the same computer, but they must inform that to admin and there won't be a problem, unless both teams make transactions eachother.
The IP is not necessarily a way o detect cheaters........ some ISP provides static IP and others use dynamic..... so, some people can have 2 accounts by just reseting their modems and getting new IP, it happens to me BTW, i have static IP at home and dynamic at work, so there's no way to do that just looking thier IP, i think that the "report cheater" is the best tool they currently have, and it depends of every user, everybody must do his part to make the game fair enough.