Problems in Joining two sculpted object I find a big problem in joining two different sculpted objects in one.
As you can see from the image below i sculpted the head and the body of a human figure and i tried to join their geometry together with the boolean modifier(union), but the result is definitely a mess.


As you can see it creates a kind of fracture due to the intersection of the two parts. You can see it better from the x-ray mode:

I don't understand why that happen, the boolean modifier always joyn two objects deleting the rest. Why it doesn't worked in this case?
The problem is also that it doesn't allow me to smooth the outer part with the brushes in order to fix at least the exterior problem.
In that condition i will be able to create a good retopology and after to project with the multiresolution modifier ?
You should use voxel remesh in blender 2.81, it is quite good for joining two separate object. First join the head and the body using Ctrl-J, then go to Object Data tab in the properties panel and under Remesh click Voxel Remesh. 



More information:
https://docs.blender.org/manual/en/latest/modeling/meshes/editing/retopology.html