The reason the F-18, F-15, F-14 and various MiGs and Su's have twin fins is so they will have yaw stability at high angles of attack. The wide fuselages on these twin engine planes can blank out a single, center mounted vertical stab. At high AOA the outside sees most of the airflow.

The single engine F-16 has a single vertical stab, but you'll notice, it is a very tall one so the tip is in good air at high AOA.

The F-111, which I flew, with its short, single tail, could get into a position of high AOA such that the vertical tail was completely blanked out and it would swap ends. These "wide" fighters needed the twin tails to cure this problem.

I had an RC biplane several years ago that could get the nose up very high on landings if you tried. It would get into a position where the fin was blanked and any gust or disturbance would cause it to swap ends about a foot in the air. A taller tail cured it.