Installing Parallels 9 on second machine disables first machine??

I've searched forum but cant find information on this:

I installed Parallels 9 and Windows Vista on my Mac Air so i could run MS Project. All works fine.

Then I installed the same Parallels 9 on my wife's Mac Pro so she could run Sage on Windows 7. I used the same disc and when prompted the same activation key on the second machine as the first - expecting that it would tell me that the code was invalid and requesting that i buy another code online, which i am very happy to do - but it accepted the code.

So i go back to my first machine, and when i try to run parallels it tells me "The Virtual Machine is not Available"

So two questions:

1. Is this blocking/clearing of the first machine a result of me using the same code on a second machine?
2. What is my best way forward - is it to start again on the first machine and install with a new code (bought from Parallels online)??

Thanks