You are correct in saying they would have two metal detectors instead of one, however you also need to consider the path(s) that the coils would trace over the ground.
For the sake of example, let's assume that a normal shoe is about 4″ wide and that you are going to metal detect a direct path from one end of an American football to the other. Using your metal detecting wing tips you would cover an area of .333' x 300' with each shoe, for a total of 200 sq. ft. using both feet over the length of the field.
On the return trip, you pull out your trusty metal detector. Walking the 300' distance while swinging a 180 degree arc in front of you would result in a sweep width of 7.5' (as measured by me, 5′11″ with normal length arms and a common metal detector) for the entire trip, resulting in a total coverage of 2,250 sq. ft. — over 10 times the area covered by the new and improved Strollamatic metal detector. Someone from the Ministry of Silly Walks might manage to improve on the 200 sq. ft. calculated above, but it would take some hardcore strolling to catch up to the old fashioned way.
If you do decide to try it, PLEASE post a video for our collective enjoyment… 😉