Hi Karen,
You can run two copies of PRO, you just can’t have both registered with the same license which prevents you from running auto updates. But everything else will work. My suggestion would be use the registered copy in the test site so you can auto update there, test things as needed, and push your changes when done.
As for the error message you’re seeing, I would suggest manually reinstalling both plugins if you’re still getting that error. You can download a copy of PRO above from the Account Central > Downloads menu.
I hope that helps but let me know if you have any questions.
– Jonah