Run Windows 7 on two different computers from only one install
Thanks to some enterprising enthusiasts, one option at your disposal is to enable concurrent sessions. For those of you not familiar with the term, here's a simplified explanation. It means you can have more than one user logged in to Windows at the same time, and neither one ever sees the other one's desktop.
It's done via RDP, the same way fancy (and expensive) MS Terminal Server setups work. This solution, however, has been generously made available to the public for free. Just download the Concurrent Sessions Enabler zip file (f you don't see the link, search for download here). Right-click install.cmd and choose run as administrator from the context menu.
Command line arguments are available: -multi allows multiple sessions per user, and -blank allows users with blank passwords to sign in via RDP. To utilize them, just create a shortcut and add them to the end of the target.
Obviously, there are some security risks with this especially if you allow remote logins with blank passwords. Make sure you've done your best to secure your network and PCs (which would likely involve not having any user accounts with blank passwords) before enabling concurrent sessions.