B Superposed loads (gunpowder and bullets) igniting simultaneously in a barrel

[manmade]

Look at this picture of two superposed loads:

Imagine that each bullet weight 150 grains and each gunpowder charge 50 grains.

If the rear load were to be fired first, it would be equal to firing two bullets +the weight of the front gunpowder charge. So a 350 grains bullet with a 50 grains gunpowder charge (if the front gunpowder charge would be compressed by the rear bullet, it might make the rear bullet slam into the front bullet which would then act as a barrel obstruction?).

But if both loads were to be fired simultaneously, will the pressure from the first load prevent the second bullet from escaping, or would it be equal to firing two bullets (300 grains) with one 100 grains gunpowder charge?

 

[Baluncore]

But if both loads were to be fired simultaneously, ...

The first ball would travel faster than normal, because it was backed by the second ball and propellant.
The second ball would travel at a slower speed than normal, because the powder ahead of it would reduce its acceleration.

 

[Nugatory]

As phrased, this is going to be hard to answer because so much is going to depend on the seriously unpredictable details of exactly how the combustion propagates within the two charges. (And in the unlikely event that you are considering actually trying this.... Don't. These details mean that the outcome is also seriously unpredictable - bursting the barrel and killing someone even after calculations show you have plenty of safety margin is a possibility).

You might try restating the problem with the explosive charges replaced by compressed coil springs - the behavior will be more predictable and easily analyzed.