as I see it there were two uses for carronades;
when they first trial'd them on frigates they used a single ball, (and were found to be poor against ships their own size ref the Essex or rainbow)
when fitted on liners (namely the 64 nade) they had a ball and a bucket of grape, until canister came out. (ref ships like the victory, which fitted them on the forecastle and poop)
I have seen no references to the loading of double shot, because as the carronade barrel was lighter, the charge required to propel 2 balls would probably explode the gun.