You do need to enable the Separate Process setting in order to fully utilize multiple cores. The main Poser executable is a single-threaded 32-bit application and though your operating system will spread the work across multiple cores, Poser will not be able to fully utilize the processing capacity of multiple cores because it only supports a single thread. FFRender.exe supports multi-threading and will fully utilize multiple cores.

Also, regarding setting the number of threads to 4 with a dual core processor: This only improves performance when the processor supports Hyper-Threading. Old Pentium 4 dual core processor and the new i7 processors from Intel support Hyper-Threading. Core 2 Duo processors do not.