You can breathe and read this at the same time. Your heart is beating and you are driving while listening to the radio. Surely you can do multiple things at the same time.
In a way you can but when it comes to forcibly paying attention, your brain can only process one thing at a time.
In the book, Brain Rules, John Medina explains that when we shift our attention to one thing to the other, the brain goes through four stages.
1. Shift alert. This where blood rushes to the anterior of prefrontal cortex, alerting the brain that it is about to shift attention.
2. Rule activation For Task #1. A two-part message where the first part is a search query for neurons to do task #1 and the second part is to excite the neurons into action
3. Disengagement. The brain needs to disengage from task #1 and the brain's switchboard is consulted, alerting it that another shift is about to occur.
4. Rule Activation For Task #2. Same as no 2.
All this happens in tents of a second,pretty fast but it is still in sequence. Not at the same time. Ergo, you cannot multitask.