How can I sculpt multiple objects to fit together? I am attempting to create a model of the prostate as four different objects that can combine together to form the complete prostate. In the end, this model will be 3D printed and will be disassembled to show the parts of the prostate. The four parts of the prostate are color coded in the diagram below.

I have modeled the blue area already as the lower model in the image below. The object floating above it is going to be the green area from the diagram.

My question is this: how can I model the three other areas around this central one so that they can all fit together in the end? I have tried using the boolean modifier, but the route always ends in disaster. Recently, I tried using only sculpting tools, but getting the objects to have a tight fit with each other is nearly impossible. I'm convinced there must be some feature specifically intended for situations like this, but I have yet to find it.
Boolean is a solution, but you could select, duplicate and separate a part of the first object (here the topology is messy because I've sculpted it with Dyntopo but it would be the exact same process with a clean mesh):


then extrude and sculpt (or model). Make sure that you don't alter too much the bottom so that it continues to fit well: