I am interested, but can you show us a rendering of how the private keys would work? Is it a piece of metal in the coin that is engraved on both sides? How would the coin be damaged retrieving the key?
I will say some simple stuff here to try make it understandable. Lets say you start with a coin and chop it in half through its center so you have 2 thinner coins. You then engrave a private on one side and some other characters on the other and then stick them together. You can only read the key if you part the 2 halves. This shows you the concept of the private key security.
BUT ITS NOT THAT SIMPLE:
It isn't simply a coin chopped in half it requires a lot of machining on a metal lathe. So to make one of these coins you actually need to start with 2 coins to make only one. From 2 minted coins you end up with 1 finished product and some scrap metal which goes towards the machining. The actual visible cut/seal line just inside the outer ring of the coin making it invisible to the naked eye. The 2 parts of the coin are sealed securely in such a way it is impossible to part the 2 pieces with out causing visible damage to the finished product.
Does this answer your question?