Can two users simultaneously share one pc

Yes. It's called multiseat. There is software for both Linux and Windows (2K, XP, Vista, probably 7) to handle this. I think Fedora 12 is aiming to ship with support. That's all I really know (and don't ask me how... I got to reading about this months ago).

Yes, this is doable using the software the the linked Wiki article passes. According to Microsoft, however, this is a clear violation of the licensing agreement on Windows client operating systems (XP, Vista, 7) and violations could trigger the invalidation of your license. Though it would not be a violation of their server O/S's (2003, 2008). If you ask the companies behind the software, they disagree of course. So it is a matter of considerable debate.

a program called SoftXpand should do the trick for what you are looking to do im not sure on the cost since they dont appear have it on their website but you find it at http://www.miniframe.com/