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That seems correct, but if You also select "render in a separate process", then to load both cores during rendering we should set the number of threads to 3.

I don't know how you get to this conclusion, but as the number of threads just refers to the number of threads used by the rendering engine, there's no change neede when changing from internal rendering to "render in a separate process".

Of course more than 2 threads may work faster, depending on the scene, because Poser assigns the render buckets to a process at the beginning of the render instead of doing it dynamically, but that has nothing to do with using "render in separate process".