You can open multiple VTF files as single AgileDesigner instance from Windows Explorer or Desktop using ARD 2.6. However, one file will be opened as 2 tabs.
For example, filename is [Reproduce.vtf] and saved on Desktop. When this file is double clicked, two [Reproduce] tabs are opened.
On the other hand, if there are some multibyte characters like Japanese in filename, this problem is not occurred.