That shows a freshly loaded script with no indication of Launch or Rename. Without having the windows renamed, it can't find where to send the commands.

Code:
<SendWinM Diablo2> failed: Could not find window "Diablo2"
I tried changing the pathway to this line to
<RenameWin "C:\Program Files (x86)\Diablo III\Diablo III Launcher.exe" Diablo1>
<RenameWin "D:\Diablo III_Backup\Diablo III.exe" Diablo2>
but it seems it didn't work.
I'd need to see the actual script you're using, but I'd probably suggest you use RenameFromPath, as seen in this script, if you're launching from different directories. In that example, be sure ScrollLock is On, launch both copies of your games then hit the associated hotkey {in the above example, Ctrl U}. The windows should now say Diablo1 and Diablo2.