Seems a bit strange that your getting longer rendertimes for a multi processor machine, the only thing that I could think of is to do as neostrider suggested and have use all available processors checked.
If your rendering with mental ray in the render->batch render option box theres an option to use multiple threads set this to 4 (as I think thats how many threads a dual core dual processor machine has) and give this a go.
Incedentaly on my dual core PC its a lot quicker than my old single core PC (althought the clock speeds of my old PC is slower)
You can varify if maya's using all the cores by opening the task manager (if your using windows) and then seeing if all the processors go to 100%
"No pressure, no diamonds" Thomas Carlyle