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You might find it easier to use the Virtual Desktop Manager, available free as part of
Microsoft PowerToys for Windows XP.
Gives you multiple desktops that you can swap between. Might be less fiddly than using fast user switching.
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I don't think that will serve the purpose - he needs separate Windows VMs/sessions, surely, per Terminal Services/user switching? Separate desktops wouldn't let you run multiple instances of an application that doesn't ordinarily permit multiple instances on a single Windows session.
What aspects of the user profiles do you want to share - Favorites/Desktop files/My Documents folder? There are ways to achieve most of these, but not (AFAIK) a single way to achieve them all with one change.