Copying art from one file to another. I right click the art, copy cover. Then on the other file to ADD this to the art work already there pressing ctrl replaces all the art work, but using the shift key does add the art work.
Correct me if I’m wrong here, but I thought was the ctrl key that was settled on?
I'm not sure if I fully understand the process you're describing. To copy cover art from one file to another, you can use right click, Copy Cover and then on the other file, right click, Paste Cover.
Another way would be to use Manage Covers... where you can apply the cover that's included in the source file to all the selected files.