Oh no, the operating system will do the best it can to share the load among the available cores.
Two single-threaded computing loops can execute independently at almost 'full speed' in a dual-core machine. I say 'almost' because while they run in independent processors, they still have to compete for other system resources.
I wonder what kind of performance I would get with a mobo that allows duel processors for these duel core babies? Imagine, a computer with two duel-core processors! Then again, if nothing takes advantage of this, whats the point, eh?