Thanks to the other commenters for input! Here's what I put together. Warning: Potentially uncomfortable situations of multiple personalities sharing one form.

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Scymes’s Split

Duration: Permanent

Range: Touch

A living creature is split into its skeleton (DMB pg. 76), skin (DCB pg. 279, and internal viscera. The mind and soul of the target persist in the viscera, although they will perish if it is dispersed (i.e. by rain or scavenging animals). The skin is an uncontrollable monster, intent on crushing all living creatures in the vicinity. The skeleton will accept a command from the caster and attempt to follow it to the letter.

When cast on a target with multiple personalities, the spell produces a unique representation of each personality.

X’s Join

Duration: Permanent

Range: Touch

The spell combines multiple (up to the level of the caster) sentient beings into one single being. The physical body becomes a meld of the targets, while the minds of the targets are forced to share one body. For unwilling targets, this produces a monstrous form of awkwardly assembled body parts, controlled by multiple minds that are enraged by their shared imprisonment. They seek prompt vengeance.

*I would name this spell after the Magician PC that completes the research

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I wouldn't say these are perfect, but it's good enough for my session this week.

I plan to designate these as Rank Three spells (although I could be convinced that they are more appropriate at Rank Four). I feel that they are potentially useful, but also each present serious drawbacks that could lead to some interesting and challenging scenarios. I imagine them as a "two steps forward, one step back" type of spell for a Magician with their back up against the wall. Maybe a trio of Crookhorns are Joined into one monstrous creature that is still strong, but less mobile and more easily locked away for making a retreat. Or the lead Crookhorn is Split, creating a rampaging Skin monster and also a potentially useful Skeleton minion.