@jimmysong's reply below is correct, FROST with trusted dealer would solve your use-case here. 

Of course  @waxwing'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  @jesseposner's FROST implementation here

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

 @AVERAGE_GARY 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.