The reason it is two pieces is for the bolster to show more separation.
When I make them (marine application), the small bolster is separated only by a piece of material between the two cushions, this material is stapled to the substrate to pull the seam that separates them into the crack between the two sections. So the zippered small bolster box is just to add to the aesthetic.
If all you are doing is going to a more firm density, you can be +/- on your new foam with regard to absolute precision as the fabric will form the foam.
Use an electric carving knife to cut the new foam.
Spray silicone will help along your cut line if you have some.
Wrap it in a piece of 1 mil plastic (you can use a garbage bag) and stuff it back into casing. Pull the plastic out after cushion is in position (optional)
hth
