Here’s a screenshot of what I get every time I attempt to merge these two discs. 51 tracks become 26 tracks.
I think IvanDobsky found the secret in that screen with Medium 2 and arrow keys, but I don’t know where to find that screen. When I wanted to identify some medleys, I had to use arrow keys to set the order of the songs in the medley. MusicBrainz would not let me type the song’s number directly, which is what I have done with the merge function. With medleys, the arrow keys are in plain sight. With release merge, they are not apparent, and the instructions do not mention them.