I tried the same technique with Visual Studio 2010 with CTP3 Denali with a Tabular model. I created a copy of an existing project but, as soon as I opened it, I got the error “This model is already open in another location by the same user, please close the other project before working on this one”:
The reason for the error is that, copying the project folder, I copied the user settings file too, which contains the workspace database definition. Because the workspace is already in use by the previous instance of the project (which is still open, due to the way I work) and a workspace cannot be used by more than one active project. For more information about the user settings file and the behavior when opening a BIM project in Denali, refer to this very comprehensive post of Cathy Dumas.
The solution to this problem is straightforward (once you learn it, thanks Cathy for explaining this!): just locate and delete the user settings file (which, by the way, is a hidden file, thus you will need to show hidden files before searching for it). As soon as Visual Studio opens a project with no user settings file, it will create a new workspace, detaching the copied project from the original one, and I can work with the two projects side-by-side.
Tabular projects work in a different way, when compared with Multidimensional ones, we just need to learn new tricks, as always.