Kill two birds with one stone

If there is one thing I love, it is efficiency.  Nothing is closer to perfection than the balance that comes from perfectly organizing my activities so that the total time taken is minimal.  Just last week I had to run two errands, and the route I chose to get from one place to the next ended in the least amount of time taken to complete the task that I deem possible in that situation.  It was perfect!  It was four o'clock on a Thursday before a three-day weekend, so the traffic could have been horrific.  And yet everything worked in my favor.   Even with two erroneous choices made while driving on the highways, I still made terrific time.  How is this "killing two birds with one stone?"  I accomplished two errands with only one car trip.
According to wisegeek.com, the adage refers to a hunter's skill at killing two birds using a slingshot and only one stone.  That definitely would take great skill.  With all of the tasks and requirements that I need to accomplish, I view this adage as one that only practitioners of effective time management can accomplish.  Interestingly, the adage is over four hundred-years-old and actually had a negative connotation.  In our ever-busy lives, it seems that killing two birds with one stone could never be negative.  What do you think?