Probably the intent is to utilize hyperthreading, in which the idle paths of a core are used to run a second thread.  Your processor must be hyperthread capable to do this.  I don't think this will work well in rendering, however.  Hyperthreading works well for running dissimilar threads, but two render threads would both want the same core pathways, so I don't think you would gain much speed.  Further, the extra threads each demand a share of the available RAM, and that is significant if you use a 32bit OS.

On my 32bit boxes, I will render four threads on a quad core if the scene is light to moderate in pixel dimensions and complexity.  If I need to render a wallpaper, I will set it to render two threads so as to allow each thread more memory.

On the 64bit machines, I'll run all the cores I have, hyperthreaded.  I made a 2560x1600 pixel wallpaper (final quality settings, ray-traced shadows) of my Ghostbuster proton pack this morning, and Galadriel pulled 9.3Gb!

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5