Sounds good. If the first ball has a 'less than sealing patch', the air can escape out the breech, as long as the rifle's modern-type system isn't a sealed unit as the air pressure must be allowed to escape out the nipple or whatever system the gun has.

The other way is if the second (top) ball's patch doesn't seal, it can allow air to escape past it's sides and out the muzzle.

It is a very real concern and even only 1/2" to 1" of air space between the balls, or between the balls and the powder can turn the gun's breech into a frag grenade.