I think Bill , while you may have a point in what you say , there are a few things to consider .
The front projectile is going to move with very little force . Really no more then what it took to seat it .
There fore the projectile will move well before the entire charge has completed it combustion.
As the projectile moves , it also enlarges the combustion chamber.
Also , even though there will be some back pressure applied to the rear projectile , any real force would I believe serve to possibly enlarge that projectile. The with pressure is it will seek the point of least resistance , that point is going to be forward in the direction of the moving projectile.
So the rear projectile would act something like a fence in a shape charge .
So unless the rear projectile a lose fit , the chance of back pressure finding its way to the rear charge is probably slim to none .
So the real question would be how much gap would result in enough heat and pressure getting to the back charge .
Well if we look at the old Nock pistol designs with a rotating gate we see that even a hand fitted gate that’s properly done , will not allow the gas to get to the 2nd charge even though one would think it would
But what actually happens is the heat and gas runs to the point of least resistance even though its surrounded the other charge .
Even the application of revolving cylinders with charges in them is a very old design which dates well back to flintlocks that had pans that were loaded on the cylinders itself . Small gates were then closed over the prime and the cylinder turned by hand . One would think the flash and resulting back pressure from the vent would ignight the other primed pans on the cylinder