A Practical Experiment
Take a small rectangular piece of paper. Hold on the two sides by length by your both hands and try to apply equal force to outwards by both hands. If it is possible by you to apply equal forces by both hands then the paper should tear into two equal sized pieces, which is practically impossible as your brain can not focus equally on your both hands and equally manage the force exerted by your hands.
Adept-ability and Human multitask-ability
There has been much research on Human multitask-ability. Some research suggests that the human brain can be trained to multitask. One study was conducted to train seven people to perform two simple tasks, either separately or together, and conducted brain scans of the participants. The persons performed poorly at the beginning but, after training, they were able to perform the tasks simultaneously quite efficiently.
Brain scans of the participants indicate that the prefrontal cortex (resides in frontal lobe of brain) of your brain quickened the ability to process information and enabling the individuals to multitask more efficiently. However, the study also suggests that the brain is incapable of performing multiple tasks at one time, even after extensive training. This study further indicates that, while the brain can adept at processing and responding to certain information, it cannot be multitasking in a true sense.