The Master
It looks like 2 or more people could register as owners of a sound recording copyright with the copyright office but they both have to be authors of everything. This isn't super clear
"If the sound recording was created as a work made for hire, then the employer or the party that ordered or commissioned the work is considered the author, rather than the performer and/or producer who actually created the recording. If the sound recordings were co-created by two or more co-authors, you must identify each author in the application."
"You should not list all of the members of a performing group, unless all of the members co-created the sound recordings that you plan to register. For instance, if Bingo, Mick, Paul, and Keith are members of a band, and if Keith and Bingo performed and produced all of the works being registered, then Keith and Bingo should be named as the co-authors. By contrast, Mick and Paul should not be named in the application, because they did not contribute performance or production authorship to the works being registered."
"First, all of the sound recordings must be created by the same author, or the works must have a common author. In other words, at least one of the authors must have contributed copyrightable authorship to each and every work in the group."
Author(s) of the Sound Recordings
But the industry as a whole seems to be geared towards 1 named owner for sound recordings (masters) which I guess would explain why Metallica formed a company. It looks like royalties and such for masters are paid to a single entity, or at least this is what I think I'm seeing.
"Within our system only entities which have been identified as Label entities can be specified. If you are an Artist who owns their Master(s), within our system you will need to create a Label entity with your legal name. In order to register your Sound Recording you must specify a Master Recording owner and identify one of the owners as the Primary Label.
... you are able to split Sound Recording Master Ownership. You can do so by selecting more than one Label entity and indicate their share of ownership of the Master Recording. You will need to identify which of the label entities' is considered the Primary Label. The Primary Label will have the authority to register the Master Recording with SoundExchange."
*** SoundExchange is a global royalties collector. While this doesn't say, it sounds like only the "Primary Label" gets the royalties collected. Still looking at this.
Master Recording Owner / Split Ownership (by TuneRegistry a publishing administrator)
I found one reference other than Metallica to a band owning a master and the payments are equally divided but I'm finding legal references of owning a master as being nothing but a single named artist (Taylor Swift) or a record company.
Owning Your Master Recording: All You Need to Know