I've done this in the past, but do not recall the path to do this now.
I want to display two folders. A & B on my computer, and then display the files that are in each in the two windows. I will then drag the files from folder B that are not in A to update folder A.
Sorry my message was not clear. And I saw no way to edit after I posted it.
Each window will be displaying only the files it contains that are not present in the other window.
The pencil icon below your post lets you edit it.
Tools > Synchronize is probably what you were thinking of.