I am not a cryptographer by any stretch, but it seems to me this would be extremely cumbersome for any significant size of text. Instead, I would do a double substitution cypher where the two parties have a defined and known key. This key is used to decipher a second cypher that tells how to decipher the rest of the text.
Ring Cypher uses predefined key of first letter written. Actual cypher key starts at second letter. First line defines the cypher for the rest of the text using some substitution rule that can be predefined, or defined by the deciphered key. It is still hardly secure, but it is a simple method to use the ring without being overly cumbersome.