Yep, that is how it works. The only way to do what you want is to disable the restrict user sessions setting.
I do a lot of terminal services in my networking gig with and without citrix. Common usernames is asking for trouble in this environment IMHO (besides the obvious security issues). It plays all kinds of fun games with applications (this depends on how they are written and where settings are stored) and printer assignments.
What that setting is really intended for is either the same person logging into multiple desktop sessions at the same time (I have a customer that does this), and for APP-V deployments where you are publishing applications instead of a desktop. If you publish an app... say notepad and a user is running it, then that user wants to open the published calculator app from the same server that second session would take over the notepad instance unless this is disabled.