IMO, the gun's stock busted because it doubled, and the wood grain was not right when the stock was made, and/or the stock/action relationship had loosened. It doubled, most likely, because the second barrel sear engagement had worn over the years to the point that, when the first trigger was pulled and that barrel fired, the recoil from it dislodged the sear engagement on the second barrel and it fired, ALMOST simultaneously.
The stock can be repaired if you're willing to wait for the right person to do the job. If that right person does it you will not be able to see the repair.
