Andrea,
If I remember correctly, rendering is not threaded (I will verify with GG later when he gets back from a bike ride).
When gg does a 2 hour movie rendering, he almost always uses a local render farm. This is described in 6.4.5 BUT I just noticed that an image showing this that was in Features5.pdf was eliminated (probably by me at some time). I will send a copy of the png to you since it shows a 16 core setup for a "root" user (the 400 range ports work only for root and I have not personally tried the 1025 range for ordinary user). In 6.4.1, just follow the basic steps 1-7 and skip step 2 since there are no clients. I have not tried the Shell Script RenderMux in a long time, but Olaf fixed this for us awhile back and it was working perfectly then.