I don't understand this as threads is normally expressed per process, so rendering in a separate process would create a new process and use the number of threads as indicated. Since Poser itself only uses 1 thread unless it is rendering this entire discussion would only be about the number of threads as used during rendering, so to use it as you indicate you should set the number of threads to 1. Using 1 core flat out for rendering and having the other available for other tasks, although the process isn't fixed to a single core and swaps around between cores but it shouldn't actually load the system for more than about 60%.