The image of attempting to climb two trees with one foot braces against each trunk is painfully awkward.  It obviously would be much more efficient to climb one tree at a time.  Yet I often find myself doing something similar to my mind by reading, watching TV and listening to a family member all at the same time.  My brain is able to take in all of the different inputs into short term memory, but only a few of them get thoroughly processed and retained long term.  Much of my brain’s energy goes into switching context between the different activities. 

I am actually more efficient and effective when I focus my mind on one thing at a time.  I find that conversations are more enjoyable and coherent at the dinner table than in front of the TV, watching a game is much more exciting when I am not reading at the same time, and I remember more information from a book if I read it in quiet (or with only background music).

Multi-tasking saves time only when at least one of the tasks (like raking leaves) doesn’t require any conscious brain involvement.