Continuing the discussion from UK Newspaper “Freebies”:
The topic of twin albums (usually it is twin but it could also be more than 2), comes back from time to time.
Here are some examples of twin albums, from my twin albums tag:
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Series — Linking those release groups sounds like the use of series but it would be somehow a pity to have lots of 2 item series… but maybe it’s still the better choice!
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Relationship — Maybe a relationship would be nice.
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Merge — The other alternative is to merge those release groups, like in this Edit #51620942 - Merge release groups which made me eventually create this topic, here, to ask for more opinions.
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Other — Please comment.
Twin albums like the mentioned ones are like two separate releases for me.
Nobody is suggesting merging the releases - the main question is what to do with the release groups 
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The first two examples seem fine as separate release groups. (I can’t read enough of the third to have an opinion.) The Hagelin example seems similar to the ABBA example in https://musicbrainz.org/doc/Style/Release_Group - two albums later bundled into a two-fer.
If the GnR albums are a “series”, is Led Zeppelin I - IV also?
Well, that series is implicit, it’s called discography and corresponds to LED ZEPPELIN artist page. 
風神界逅 and 雷神創世 are like the others, simultaneous release of two albums, result of a single project.
All those, not only Higelin’s, could have a compilation release later.
What’s specific with Higelin’s is that the package for Champagne LP, contained some room for the Caviar LP package inside, making it kind of a one‐package once you bought both.