[shif]

What if one of the people that has part of the key gets mad and leaves without sharing it?

[seanp2k2]

https://blog.threatstack.com/who-watches-the-watchmen-securi... // https://en.wikipedia.org/wiki/Quis_custodiet_ipsos_custodes%... this idea has been around for a very long time.

[livingparadox]

Then you still have two users with a complete password set, from which you can either reconstruct the third permutation or generate a new set.