Quote - Carrara renders the same scene twice as fast with two cores.
You're now probably going to tell me that that is because one core can only access the RAM with half speed...

Have you tested it with the same computer or the renders were made on two different computers?
It will also depend on the scene content. If you have a scene where no textures are used with exception of the background, a scene with only procedural shaders, this scene will render faster with many cores.

Stupidity also evolves!