nostr:npub10vlhsqm4qar0g42p8g3plqyktmktd8hnprew45w638xzezgja95qapsp42's reply below is correct, FROST with trusted dealer would solve your use-case here. 

Of course  nostr:npub1vadcfln4ugt2h9ruwsuwu5vu5am4xaka7pw6m7axy79aqyhp6u5q9knuu7's point is also valid: If you want true multisig security you'd need to be certain that you securely erase the distributed shares on your machine afterwards, and of course erase the original secret key (except for maybe keeping a paper backup). Still, it's a valid approach if you are OK with those assumptions.

I'm currently working on a BIP340 compatibility pull request into the ZCashFoundation's FROST implementation (Rust), I believe it should work to produce valid Nostr sigs.

https://github.com/ZcashFoundation/frost/pull/730

If you use a python stack, check out  nostr:npub1tr7ndmjkguzupvqhn9clasx4r08q2sklhal4g60844nsnnalw0tqpwnc05's FROST implementation here

https://github.com/jesseposner/FROST-BIP340

 nostr:npub160t5zfxalddaccdc7xx30sentwa5lrr3rq4rtm38x99ynf8t0vwsvzyjc9 linked to my FROST BYOK article but that would only be practical if you wanted to create a joint key from existing (static) npubs. I don't think it's useful for splitting an existing key.