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You can split up a key using something like https://en.m.wikipedia.org/wiki/Shamir%27s_Secret_Sharing

1. Key is 256 bits K

2. Half of K is a random 256 bit X

3. Other half is (K xor X), still 256 bit

Having half of key is still 256 bit bruteforcing.

You could instead construct a key by appending two securely-long passphrases together (which will then go through a KDF in any good encryption software). Give each passphrase half to one person. Recombining them is as simple as typing both of them into the passphrase input in the decryption software.