Goku Black and Zamasu split because one of them wasn't a Kaioshin.

[Desassina]

Shin and Kibito didn't split, but the latter was an apprentice, because the old Kaioshin didn't as well. Was merged Zamasu an oversight by Toyotaro or is there something else to be explained? Could there be a special relationship between Shin-jins not yet Kaioshins that backfired once Zamasu became Goku Black? Or is there a role above that of a Kaioshin (such as Grand Supreme Kai from the dub) that allows Shin to have become one and welcomed another?

[SupremeKai25]

Zamasu not being a Supreme Kai does not make sense. Previously he was able to use the Time Ring, something that only Supreme Kais can accomplish. It is also shown that to become a fully-fledged Supreme Kai, you only need to wear one of those special Potara earrings, there's no "unique ritual" involved. This rule applies to the manga too, as that's how Gowasu temporarily promoted Zamasu, by giving him one of his earrings. Black also counted as a Supreme Kai because he used the Time Ring to jump through time several times: first to get his wish, then to find a suitable timeline in which to enact his plan.

Therefore, Fused Zamasu NOT being an official Supreme Kai doesn't make sense in light of what was shown previously. If neither Black nor Zamasu counted as Supreme Kais, then they wouldn't have been able to use the Time Ring, and Zamasu wouldn't have been able to be promoted to a Supreme Kai just because he put on one of Gowasu's earrings to go to Babari. Black having taken a mortal body for himself shouldn't have changed anything, since mortals can indeed become Supreme Kais (see Future Trunks in the manga). Fused Zamasu should have been eternal like in the anime.

If Toyotaro wanted to convey the idea that "Time Rings are not exclusive to Supreme Kais", he did a terrible job.

[TobyS]

It's no oversight. You need one Kaioshin involved to stay merged, neither person here was a Kaioshin.

Only Kaioshin are allowed to use the time rings, but others can physically use them obviously. Gowasu states he's temporarily making him a Kaioshin with the earring, but the earring alone probably doesn't do it either.

Its makes sense fine IMO.

[SupremeKai25]

It's no oversight. You need one Kaioshin involved to stay merged, neither person here was a Kaioshin.

Only Kaioshin are allowed to use the time rings, but others can physically use them obviously. Gowasu states he's temporarily making him a Kaioshin with the earring, but the earring alone probably doesn't do it either.

Its makes sense fine IMO.

Except if anyone could use them, then the entire plot point of Gowasu making Zamasu a Supreme Kai specifically to use the Time Ring would be meaningless. You couldn't even justify it by saying "Well, it's tradition. There has never been a non-Kaioshin who used a Time Ring", because Zamasu would have used it under Gowasu's supervision and with his approval anyway.

Furthermore, if wearing the very earring of a Supreme Kai wouldn't be able to elevate you to such a position, why should the words of a Supreme Kai be enough? So wearing their very same artifacts is not enough, but a single line -- "You are a Supreme Kai" -- with no further visual indication (at least the anime gave a visual and auditive cue that Zamasu became Supreme Kai) or special ritual automatically seals the deal? Where is the logic behind that?

[ZombieVito]

Shin and Kibito didn't split, but the latter was an apprentice, because the old Kaioshin didn't as well. Was merged Zamasu an oversight by Toyotaro or is there something else to be explained? Could there be a special relationship between Shin-jins not yet Kaioshins that backfired once Zamasu became Goku Black? Or is there a role above that of a Kaioshin (such as Grand Supreme Kai from the dub) that allows Shin to have become one and welcomed another?

Zamasu and Black are Kaoshin to use the rings and the like but then they are not for staying fused with the Potara.

It's a legit plothole. Very common for that hack.

[Desassina]

I think that I figured out what the problem was, and it's not a plothole, given that Shin and Kibito remained fused. While Goku Black has the essence of Zamasu, who is a Kaioshin fusing with another, his body is still that of Goku fusing with a Shin-jin, both of them being of different races or lineage, so that the essence remained that of a single Kaioshin in bodies that split.