What Is the combined speed of two approaching vehicles?

Speed A + Speed B = Speed Combined

Unless they hit each other. Then, under Newton's Third Law, it's

(Speed A + Speed B) / 2 = Combined Collision Force

IE, if two cars hit each other head-on, both going 50mph, it's the same force as a single car hitting a wall at 50mph. (i.e. The impact force does not double; because you have to divide the absorption force of a second vehicle, which cancels out half of the force)