As Small link mentioned, for split keys multisig is the way to go. This general idea was
discussed years ago and is
very well established albeit rarely used.
Q: What if a trustee loses its piece of the passphrase?
A: You can send copies of the same piece to multiple trustees using a "split" technique:
- Split the passphrase in 2 pieces
- Send the 1st piece to mom and wife
- Send the 2nd piece to dad and best friend
This is fine but notice that a more efficient arrangement is possible with 2-of-3 multisig. 2-of-3 multisig means that you have 3 pieces, any 2 pieces can unlock, and any number of pieces less than 2 cannot unlock.
For myself, I'd prefer a locktime-based dead man's switch.