This theory has been around for quite some time, the theory that Rigdon and Cowdery had some doing in writing the book of mormon. However, the strangeness of the relationship of these two to mormonism tends to speak otherwise. Both had a fallen out with the lds church under Joseph Smith. And both left the fold. And yet, they never spilt the beans about the book of mormon being written by them. And both had good reasons to do so. And by doing so, the lds church under smith would have been destroyed.
Also, JS was reported to have translated the book of mormon by various means. One of those means was by looking in a hat and reading out the text of the book of mormon. This would have been quite a feat if Rigdon and Cowdery had written the book. This process was seen by David Whitmer and Emma Smith.
Also, how does one account for the witnesses to the book of book of mormon? None ever denied their testimony and several had no reason to keep the lie going. Even Oliver Cowdery came back to the fold never denying the book of mormon.
And so, the theory that Rigdon and Cowdery wrote the book is intriguing but the interesting facts about the book and how it came to be does not match that theory.
Not to mention the fact that emma never denied her husband’s prophethood or the book of mormon even when it would have been advantageous to do so.