I’m trying to merge documents into one document

I’m trying to do Edit/Track Changes/Merge Documents. Basically taking one original document, then Edit/Track Changes/Record Changes, then copy and paste to two different users or folders where changes were made under the Record Changes and then saved. Finally going back to the original and Edit/Track Changes/Merge Documents.
It only allows it for the first merge. The second one won’t work. This is no better in practice than using the Compare Document feature. I found one video of this process in English and have tried several times copying everything he does to no avail https://www.youtube.com/watch?v=qe-lRVpk3eA

which means ?