Specifying which version of Revit will launch when double-clicking on a .RVT file

Unfortunately it is not possible to specify which version of Revit will launch when double-clicking on a RVT file. The file will always open in whatever version of Revit was opened last.

There is an existing request open with the development team to allow control over this that is under consideration for the future.
I would also suggest providing comments and a business case as to why this is important to your productivity directly to Revit Product Design at http://www.autodesk.com/feedback.