Why the use of twin tails?
This constructive solution can be dictated by a number of reasons: 

Aerodynamics - for AN 225, 747 Shuttle Carrier Aircraft, A-10 (control surfaces are more effective outside turbulent air); 

Some sort of redundancy - for military airplanes F-35, A-10;

Easier construction - since the surface being smaller aerodynamic loads are smaller the structure can be lighter wight;

Radar cross section - F-35, SR-71;

Stability considerations: the arm for the vertical surfaces is a little longer than the arm of a potentially single surface on the longitudinal axis of the aircraft, so the two vertical surfaces combined can have a less surface area than a potentially single surface.