That just circumvents the other part of the feature; the timeout. It also means you have to trust that individual's security.
If you have a house, which has windows, your locks do not provide security against someone smashing open the window. Key cutting schemes are a bit like this - no key offers security, only one of several access routes.
Having multiple access routes may be desirable and simultaneously a concern - a fireman smashing through your window to save your life is desirable, a burglar slitting your throat after smashing through your window is not.
Encryption is more like a lockbox or a safe room - having a burglar compromise your safe room is undesirable, and going into one during a fire is also undesirable. But you do want to use one in the event of a burglary.
A key cutting scheme may be useful in the case of mutli tenancy, but it is not a reasonable dead man switch - if your data needs to be re-encrypted either the keys themselves must be related (calling into question the security of the keys), or the encrypting party must multi encrypt the data, meaning whomever does the encrypting has full access to all the key data.
If e.g. you are yourself encrypting the data, you must multi encrypt - it would be faster just to share the key yourself, as you already have all the keys. If the third party is encrypting, this means they have side channeled your data such that they can decrypt at any point.
Again, even in the case there are e.g. two mathematically related keys, you cannot then enforce a timeout without first referencing and thus controlling the original key. You MUST distribute your keys yourself to your 3rd parties, or your data cannot be secure.
No. Cutting a key in half doesn't halve its security, but it reduces it exponentially.
256 bits = 2^256 possibilities for bruteforcing
255 bits = 2^255 possibilities for bruteforcing, or half
128 bits = 2^128 possibilities, or 1/(2^128) the security
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.
256 bits = 2^256 possibilities for bruteforcing
255 bits = 2^255 possibilities for bruteforcing, or half the security of 256 bits
128 bits = 2^128 possibilities, or 1/(2^128) the security
But you can have encryption schemes requiring N-of-M private keys to decrypt.
At no point does Bitwarden the server have a copy of anyone's private key. And no splitting of keys is necessary. This is just the normal way asymmetric encryption works.
This, of course, all breaks down if you don't trust Bitwarden the company, since they provide you the client. As far as I understand, US law enforcement doesn't have the legal ability to force a company to modify their own software to make it malicious (as opposed to doing something much simpler like forcing them to turn on IP logging on a VPN server). But if your threat model includes the possibility of US covert intelligence services MITM-ing Bitwarden the company and sending you their own malicious client, then yeah, keep your secrets in a physical vault guarded by people willing to die in a shootout with the FBI before betraying you. Make sure they'll answer to your successor if you die.
It still comprises a break in the end-to-end crypto, and can still bypass the time delay and decrypt your passwords today without your involvement.