Has it ever happened in royal history where a twin with a brother succeeded -- or would have succeeded, had he lived -- to a throne? I know that the Prince of Monaco is the father of twins; but since one is a boy and one is a girl, birth order is irrelevant (the succession law being male-preference primogeniture).
Also, Prince Karl Friedrich of Hohenzollern-Sigmaringen, present-day head of the house, is the eldest son of Friedrich Wilhelm, the eldest son of Friedrich, who had succeeded by virtue of being the elder twin son (by a few minutes) of Wilhelm. But since the princely house was deposed in 1866, none of them was ever a reigning monarch.
I believe that the highest-ranking twins in the present line of succession to the British throne are Prince Philip and Prince Alexander of Serbia, the younger sons of Crown Prince Alexander. But since they are so far down in line, there is a far greater likelihood of the two being struck simultaneously by separate bolts of lightning at separate places, than of Philip becoming king.
Anyhow, I was just wondering about other instances in royal history, where there was a near or potential succession of a twin son to a throne.