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?
So how would you ever redeem it without potentially ruining the keys while cutting it open?