Curious to know why the 64 bit DXi version of Dimension (1.2) is showing up under VST Instruments and DirectX Instruments in Plug-in Manager.
 
It's the one installed in C:\Shared Dxi\Dimension LE

It always helps to know which version of SONAR is running or in this case the last version of SONAR installed on the machine since they all use the same plug-in manager and VAT scanner. From the information provided, it would seem to be a bug. Inspecting the registry entries may provide a clue. Hopefully your VST scanner does not include any parent or the actual DXi path in its scan path. Strange things can happen when the scanner is asked to scan areas which contains dlls other than VST plug-ins.

The plug-in manager scans for both. Can VST and DX versions exist within the same .dll file? Or are they 2 different files with the same name?

The plug-in manager does not scan at all. It reads entries from the registry.
 
The VST scanner (a separate program) builds the registry entries for VST plug-ins. The VST scanner can get messed up if given a bad scan path.
 
DX plug-ins are added to the registry usually during installation with regsvr32.exe.
 
A plug-ins can only be one format. The exception being PC and VST2 format. It is possible for one dll to show up in the PC and VST2 lists. This is because the PC format is based on the VST2 format.
 
Cakewalk does not install VST plug-ins in the "Shared" folders.

Didn't realize that, good to know. Makes sense now. Sometimes we're so caught up in the doing of things we don't realize what's actually going on. Removing DXi folders from VST scan list now.