FISSION
8th-level transmutation, body, shapeshifting
Casting time: 1 action
Range: Self
Components: S, M (a bisected seed that has each half sprout its own root)
Duration: Concentration, up to 10 minutes
Your body tears in half. Each half exposes organs − writhing, deforming, regenerating − to form two whole yous. The bodies stand within 5 feet of each other. Each emerges as you as if after a long rest. Clothing of some organic fabric (such as cotton, linen, wool, or leather) in any style can optionally weave around each body during the Fission. You choose which body keeps which items that you wear or carry. Both bodies express your will, and you share the thoughts and sensations of both of your bodies. But each body can do actions independently of the other, such as spell casting. Only one body needs to maintain concentration to sustain the Fission, and if one body loses concentration, the other can recontinue it as a reaction. On your turn, decide which body takes its turn first, and which body takes its own turn afterward. Track hit points separately. As long as one of your bodies remains alive, you yourself survive. When the spell ends, choose which body remains, while the other decomposes into dust. You retain the memories of both bodies, and in some sense both bodies survive in you.
Add comments that
- neither 'half' can cast this spell, only the "whole you" is able. (No chaining, nor creating your own personal army pardon the pun)
- if the halves move beyond a certain distance from each other, or lose line-of-effect to each other, you choose one half to decay to dust immediately. The surviving half knows where the other half decayed (so you can re-collect the rest of your gear).
I like how your text is pithy. We'll have to brainstorm some abuses of the spell in-play, and corrective text.
I was thinking of Team Monster Thunderwaving (or whatever) one half across the room / down a stairwell, in order to deal with them separately.And this tactic of sending one of the fissions away on a mission loses the benefit of two of you focus-firing against one hostile.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.