HI Graham,
I first want to check and make sure you are using both of your Start11 keys and not trying to use the same one on both installs.
If that is ok, please do the following on each install:
Press the Windows key and "R" at the same time. A "Run" dialog box will pop up, please paste the path below into the "Open" field and press OK.
%ProgramData%\Stardock\
In the folder that pops up, you should see a folder named Start11. Please delete this folder and all of its contents.
After you have deleted that folder, please attempt to activate once again and it should succeed without any errors.
Paul Scroggins
Associate Technical Support Analyst