But that's the "you need two keys at the same time" case, which is different. I'm talking about "each of them can lock alone, but once locked, only the other on

But that's the "you need two keys at the same time" case, which is different. I'm talking about "each of them can lock alone, but once locked, only the other one can unlock". The real problem is that lock and key is a fundamentally dumb analogy for a process that scrambles something.