As I know, In TFS2010, One Build Controller serves One Project Collection. And, ideally one build server should have One build controller into it.
However, as per the link below-
http://marknic.com/2010/05/14/MultipleTFS2010BuildControllersOnASingleBuildBox.aspx we can install mutiple build controller in a single build box.
Can two or more build controller can run at a same time.
Because the link suggests that, we need to switch between controllers ....
Is it that, we can use one controller at a time.