Engines has a good point about "twin boom" designs - on the F-22, JSF, F-15, MiG-29 and Su-2x/3x, you'd need to add beef in the middle for a single fin.
Canting for stealth is another big consideration - the vertical tail gives you a large lateral RCS spike, albeit a narrow one.
However, in the case of the F-16, F-15 and F-18 (and I am willing to bet the Typhoon and Rafale as well) the decision was more like "do we have one tail and make it huge, so that it does not get blanketed at high alpha, or do we have two and run the risk of them getting buffeted and overstressed by the vortices?"