Ahhhh- the beauty of a double, one loaded with ball, the other with shot- normal practise in India in the mid 1800's. An East Indian Proverb states "When ou go out to shoot a hare, be prepared to meet a tiger". Forsyth states (quote) "in the dense forest tracts which I am constantly traversing, even when shooting small game with a shot gun, I never load more than one barrel with shot, ...; for my spaniels have roused a panther when I expected to see them flush a partridge. I have found myself face to face with wild elephants when looking for jungle-fowl; and I have almost trodden on the tail of a tiger, when stalking a spotted buck." The 14 bore was a favourite for India.
: EXTRA ball on top of the shot charge will more than double the recoil and possibly have disastrous effect upon the wrist/tang area, depending on the fit. I wouldn't do it, even if I thought the higher pressure was well within bounds- there are some doubles I wouldn't do this in.
: Now, in a 12 bore, shooting a normal 1 ounce charge, doubling that to 2 ounces with a 1 ounce ball wouldn't do more than kick more, but the velocity would be in the 700fps range & you must practise to see what the results would be, how to sight, etc. 2 ounces overall isn't bad, but if you normally overload the shot like most folk do, you may end up with 2 1/2 to 3 ounces for the payload- I wouldn't do that. 1 ouce of shot is a very good shot load, by the way.