Creating a second background session with Apple Remote Desktop
Assuming you are talking about using Apple's Remote Desktop app and not just Screen Sharing (which I'm guessing you are, based on your question) then yes, it's perfectly possible, you just have to set it on a system-by-system basis. Oh, and it only works on 10.7+. Here's how:


Right-click the system you want to remote in to and choose 'Get Info'
If you haven't already, set your administrator credentials under 'Login' and 'Password' on the 'Attributes' tab. Click 'Edit' in the lower right to enable you to, well, edit. Make sure these are not the same login credentials used by the system's actual user otherwise you'll remote in as them and share the screen
Switch to the 'Control &amp; Observe' tab
Click 'Edit' in the lower right (if you didn't do it at step 2)
Set each radio button to 'Connect to a virtual display'


Now when you choose to 'Control' that system, ARD will spawn a new window process and connect you to a virtual display, allowing you to access and configure the machine without disturbing your user. (:

Don't worry, you won't have to set these options every time you want to connect, just the first time. I wish there was a way to set that as the default behaviour but alas.

Hope this helps. (: