This time I tried setting up two different possible PT where one would encrypt while the other decrypted and vice versa.
This works but it would always be halfway insecure and all you’d have to do is toggle it back and forth. It works but it’s mostly a mental exercise at this point. I suppose if you did an substitution on the 2nd bit and then it would be splattered all through the other one but still doable.
I guess the only way to tell is if you can keep K3 and K4 together and decrypt the one while the other lingers on as an unwanted house guest.