[KMag]

You misunderstand. This has nothing to do with comparing two torrents. Comparing two torrents solves the wrong problem.

Clients that have downloaded all of the data for a single file (but may or may not have downloaded all of the data for the full torrent) have the data for the file and can calculate the Merkle tree root for that file, and advertise availability on the DHT.

Clients with new style torrent files that included Merkle tree roots in file descriptions would then be able to download those files. This has nothing to do with comparing torrent files.