You’re right, I had some other people in mind, I should have written this advice somewhere else, sorry. 
Back to the topic, the guideline about SACD layers as mediums is in Style / Release - MusicBrainz but if the tracklist is the same, it is not really necessary to add 5.1 tracks.
I found it from the guideline proposal topic, where the reason is given by @Freso:
The CD sections of [Hybrid SACDs] support DiscIDs, and you cannot have a medium with more (or less) tracks than what’s defined by the attached DiscID.
By the way, your merging release does have Disc IDs, so you will not be able to add those tracks, anyway.