Buzzcook, if you truly have read of 2 bores being used in either smooth or rifled I and my fellow researchers would love to know where. This is NOT a "challenge" at all. I am 100% sincere in asking to please show me such documentation. We need it if the book is ever to be updated or edited to that effect.
After about 10 years of research I have yet to find one record of a 2 bore or 1 bore being made in anything but swivels guns and whaling arms.
In sporting rifles styles I have found a few records of 4 bores, but never anything larger.