David Rock in his book Your Brain at Work, clearly explains the limitation of working memory capacity of the new rational human brain i.e. Prefrontal Cortex
As a result of this trying to perform more than one conscious task at a time can be detrimental in terms of performance.
Yet few people effectively multi task.They practice and practice to create habits out of mundane tasks e.g driving a car.The habits sit in long term memory i.e. Basal Ganglia and have nothing to do with working memory.This is precisely the reason that few have mastered the art of driving as well as taking conference calls at the same time.