AutoCAD Mechanical 2011 takes 2 licenses

Here is what Up and Ready says about this:

If you have Autodesk Product Design suite Premium 2012 Package license and if you still use 2011 version of AutoCAD Mechanical and Inventor, you might find that it is using 2 seats (if you have more than 1 license) or for you get license error for the second product.

One work around to the issue is to make sure the Inventor 2011 is started prior to starting AutoCAD Mechanical 2011.

If you don't want to pass on the responsibility to the user then you can create an options file with the following line to exclude AutoCAD Mechanical 2011 to all the users, forcing the users to checkout Inventor 2011 feature code.

INCLUDE 85545AMECH_PP_2011_0F USER DUMMYUSER