Reason I asked is that I prefer to give half a key to Alice and Bob and the other half to Charlie and Dennis. I consider this a backup strategy in case something happens to me (so my family can inherit) or I lose my notes (e.g. house burns down). To add safety I could use multisig, but the risks to worry about should be elsewhere; may these conspire and steal my coins, did a screen capture see the private key, did I write them down without any typos?
Well I wouldn't write down any secrets by hand and your additional risks always apply not matter how you share access. The best solution would be to store the funds in a P2SH address which requires m of n (i.e. 2 of 4) private keys. If you wanted to ensure certain subsets are require (i.e. (Alice or Bob) AND (Charlies or Dennis) ) then you could just make it 2 of 2 and give each "or" pair the same key.
Instead of P2SH another option would be to SSS (Shamir's Secret Sharing) to break a single key into n pieces where m of them are needed to recreate the original secret (the private key). Honestly I don't see much use in this but it could be done. One last option which is simplistic but still provides better security than cutting the key in half is XOR two keys to produce the final key. With one of the two subkeys the security of the key isn't reduced. The two subkeys should each be 256 bit but here is an example with smaller keys.
01010110 Produce random subkey1
11000001 Produce random subkey2
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10010111 XOR the two subkeys to produce the Bitcoin privatekey
Giving one person 01010110 and the other one 11000001 and telling them to XOR their keys to produce the full key is better than giving one person 1001 and the other one 0001 and telling them to concatenate the pieces.
No matter how you split the "secret" you could enhance security by delaying the funding of the address by making the funding transaction have a nTimeLock set years in the future and providing all participants with that tx. Even if your friends conspire the address is empty until the nTimeLock is reached (and they can broadcast the funding tx). You obviously don't know when your death will be but this can be extended as many times as needed by spending the original coins to a new address (redning the nTimeLock tx invalid) and creating a new timelocked funding tx.