If the electrician can attach a rope to the shorter one so that while moving the end of the longer one to where the connection is to be made, he can retrieve the shorter one to that location (or vice versa), then the connection can be made 12 feet below the ceiling, with the two cords pulled taut to form the hypotenuses of two right triangles with a common 12-foot leg.

Then the arithmetic is

>> sqrt(16^2-12^2)+sqrt(18^2-12^2)

ans =

          23.9994131092571

          

so that the separation of the two cords can be almost 24 feet.

If the electrician has no rope he must bring the longer cord to where the shorter one is, and the hypotenuse of the single right triangle formed is the 18-foot length of the longer cord and the vertical leg is 16 feet, and the arithmetic is

>> sqrt(18^2-16^2)

ans =

          8.24621125123532

          

or almost 8  1/4 feet.          

Of course the electrician also has a span between his left and right hands, so he could expand a little on this, maybe something like 5 more feet.