With regard to the puzzle idea: if the secret is quite sensitive, and assuming you split the card in say 4 parts, then coercing 2 of them to reveal their puzzle pieces, would reduce the brute force security of the secret from 128 bits to just 64 bits, which could be easily breakable.
With regard to the superimposed cards idea: this is equivalent with an "and" operation on bits, which in terms of entropy means that having one of the cards would reduce the brute force security by the amount of "0" bits (i.e. non holes in the card), because those bits can never be "1" due to superimposing.
The only safe way one can *easily* create multiple shares is through bit-wise XOR-ing:
* generate two random number, encode them into the cards, the actual secret is the XOR of the two numbers;
* K out of N is possible (I've seen some articles / StackOverflow posts), similar (though not quite exactly) to how RAID5 handles data;
With regard to the superimposed cards idea: this is equivalent with an "and" operation on bits, which in terms of entropy means that having one of the cards would reduce the brute force security by the amount of "0" bits (i.e. non holes in the card), because those bits can never be "1" due to superimposing.
The only safe way one can *easily* create multiple shares is through bit-wise XOR-ing:
* generate two random number, encode them into the cards, the actual secret is the XOR of the two numbers;
* K out of N is possible (I've seen some articles / StackOverflow posts), similar (though not quite exactly) to how RAID5 handles data;