Someone entered it on MusicBrainz as 2 separate single-CD releases
Here are the problems that I am facing:
1-Those releases are affiliated with the master releases of the individual albums, not with a master release for the 2xCD compilation.
2-Those entries have Disc IDs
3-How do I merge the two 1-disc entries into a single 2-disc entry, keep the Disc IDs and create a new master release for the compilation?
Hit ‘merge’ on both releases. Follow the steps to ‘append mediums’ and save.
Edit the release you chose as the ‘target’ in the step above, and click the magnifying glass in the ‘Release Group’ field. Scroll to the bottom and click ‘add a new release group’. Fill out the details and enter edit. Save your edits to the release.
You don’t have to worry about DiscID’s, they will stick with their mediums when the merge happens (as far as I know). I always check back after merges pass/after the voting period in case anything weird has happened (e.g. label double ups or similar).
If any of that doesn’t make sense let me know and I’ll post some gifs