I'm not sure that you can do this. You can set up different user accounts and turn on fast user switching. If you've connected a 2nd monitor then your machine should auto-detect it and you may be able to do it via the extended desktop option but I've never tried so my advice is probably about as much use as a chocolate fire-guard! There are some very experienced guys on here and they may have a solution.
I don't think you can physically run 2 different accounts at the same time. The only thing I can think of is having 2 machines and use screen sharing to log into the first one with one user account while it runs a different account on the screen. I do believe though that you'll need Lion to do this - Older versions of ScreenShare don't support logging in with a different user than currently logged in.
...I don't think this option is very economic. If you have 2 machines you might as well run those and share files instead of sharing the screen
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