Basically write the private key on a piece of paper. Give it to the other fucker =D
Problem is, must forget about the private key, don't copy it !
One possibility comes to mind:
Some kind of service... that generates private keys in an encrypted form.
Give the password to the other fucker... give the private encrypted key to the other fucker.
Send money/bitcoins to this key.
So everybody remain blind, except the guy getting the private key...
He must then be able to decode it, have the password... Hmmmmm...
Homomorphic encryption might be a solution for this, or maybe not...
And otherwise seperate the encrypted private keys from password key into different channels, so that none of both... might help somewhat
Finally a technical solution:
The private key could stay encrypted. Using homopathetic encryption.
The password key could be given out... as well, possible also encrypted.
Both are then used to perform operations on the private key in a homopathetic encryption method.
Thus the only thing required is the possesion of both.
To prove to the system that both are in possession zero knowledge proof could be executed based on both possesions.
So that decryption of private key or password is never truely necessary.
Just ownership is enough...
However to be able to actually do anything with it, requires owner ship and plugging it into some system to ultimately use it with bitcoin system ofcourse...
However if such technology available, then both could be copied... but one must get access to both...
Little bit interesting.