Sharing arcana

PCs can share arcana with each other, or even with NPCs, as long as it makes sense to do so.

If the PCs share a minor arcanum with each other, work with them to figure out how the requirements might change.

The next winter, after a year of strife, Caradoc asks Vahid to teach him the Thunderous Bellow spell Thunderous Bellow. Vahid agrees! They've already met the first two requirements (digging up the plaque and deciphering the runes), but it'll still take a few weeks of study and Caradoc still risks harming his voice, hearing, or neighbors as he learns.

"Can I Aid?" asks Vahid. He sure can in fact, he needs to. Caradoc can't read Maker runes! Vahid's Aid is allowing Caradoc do more than he could alone; it doesn't grant advantage when Caradoc Defies Danger.

If the PCs share a major arcanum with each other, then in general:

Each character must unlock its mysteries individually, tracking their own progress
Each character tracks their own consequences individually, meaning they can each suffer the same consequence

Of course, each arcanum is different and there will be exceptions. For example, the consequences for the Mindgem all affect the Mighty Servant itself, reflecting how it learns and changes. Thus, multiple PCs giving orders to the Mighty Servant would all be marking off the same list of consequences.

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NPCs using artifacts and arcana

At some point, an NPC or monster will get their hands on a magical artifact or arcanum. The PCs might give it to them, or teach them to use it or the spell it imparts. Or, the NPC might acquire/steal/borrow it from the PCs. Or it might be in the NPC's possession to begin with.

Most magical artifacts and all arcana are written to be player-facing, often with custom moves or procedures that assume a PC is using them. So how does that work when an NPC or monster is the active party?

First, consider whether the NPC or monster can use the artifact/arcanum at all. Not everyone has the talent, learning, or aptitude to learn spells or command magical effects. Of course, some magical artifacts just work and anyone can use them.

If an antagonistic NPC or monster is learning to use an artifact/arcanum, you can reflect that as an impending doom and grim portents, or even write up the artifact/arcanum itself as a threat (a MacGuffin, probably).

If one or more friendly NPCs are learning to use an artifact or arcanum, and that's something the PCs want to happen, then Make a Plan to lay out the requirements. If their mastery of this magic would provide some lasting benefit to the village, then write it up as a steading improvement.

To reflect a monster or NPC using a magical artifact or arcanum, you might…

… just imagine how it works in their hands, and incorporate that into your descriptions and GM moves when you portray them;
… reflect it in their stat block, as one or more tags, special qualities, moves, or combinations thereof;
… write custom player moves for what happens when they use the artifact against the PCs, and/or…
… write it up as a threat (probably an affliction, a magical entity, or a MacGuffin) if the NPC's use of the artifact is going to cause long-term trouble. This is a particularly good choice for major arcana—their consequences make great grim portents!

As you do all this, don't worry about replicating the artifact or arcanum exactly. Magic is weird and fickle, and might work differently for different people or entities. Aim to capture the feel of the requirements, moves, and drawbacks, but don't fret too much over consistency.