OK. Here's the formula for a problem. Load normally with a patched ball. Load another patched ball on top of the first.
The problem is the patched balls should be an airtight fit. It is possible that the air between the two balls would compress and not allow the second ball to seat properly. Thus creating the dreaded air gap and allowing the possibility of a ringed barrel or a burst gun.
A round ball loaded with out a patch with a patched ball on top of it should be alright.
a patched round ball with an unpatched ball on top of it with a paper wad to hold it in place.