How to connect to remote system in Windows 7 when another user is already logged in

Edit:

DISCLAIMER (courtesy of Daniel Petri)

Before you begin, I need to warn you that patching the file and allowing more than one concurrent Remote Desktop session will violate a few lines in the Windows 7 EULA. Proceed with caution and at your own risk. I shall not be liable for any damage caused to you, your computer, your data or your dog/cat because of this.

There is no way to do this via built-in tools in Windows 7. There exist some 3rd party tools however, termsrv patch for example. (I used it myself)

If you decide to try this, be aware that you're doing so AT YOUR OWN RISK, this is a "hack" (if you want to call it that) of your OS, it may happen that you run into weird / unacceptable problems.