Has a twin ever been heir to a throne?

Search: have twins ever been heir

Why: On last week's "Archer":

ARCHER: When Mother found out you cheated on Lana-
CYRIL: You cheated on Lana plenty.
ARCHER: Yeah, but with starlets, models - oh, and one time, two actual princesses.
PAM: Two at the same time?
ARCHER: Yeah, they were sisters.

Answer: There have been several sets of royal twins, but I see only 1 that actually became King:

• 1430 - King James I and Joan Beaufort of Scotland - Alexander Stewart, Duke of Rothesay (died in infancy) and James II, King of Scotland [5th and 6th children, but first sons]
  
• 1556 - King Henry II and Catherine de Medici of France - Joan of Valois (stillborn) and Victoria of Valois (died at 7 weeks old) - [9 and 10 of 10 children]
• 1727 - King Louis XV and Marie Leszczyńska of France - Louise Élisabeth de France (later the Duchess of Parma) and Henriette de France [1 and 2 of 11]
• 1750? - Chief Keawepoepoe and Kanoena of Hawaii - Kameʻeiamoku and Kamanawa, the niau-pio twins on the Hawaiian coat of arms:

• 1893 - Prince Frederick Charles of Hesse [King of Finland] and Princess Margaret of Prussia - Friedrich Wilhelm Sigismund and Maximilian Friedrich Wilhelm Georg [1 and 2 of 6] (both twins died in WWI, but their dad renounced the throne anyway; see below)
  
• 1896 - Prince Frederick Charles of Hesse [King of Finland] and Princess Margaret of Prussia (same fertile parents!! [they had a third set of twins, too!]) - Philipp, Landgrave of Hesse (and Nazi Chancellor) and Prince Wolfgang of Hesse, Crown Prince of Finland:
  

• Wolfgang would have been his father's heir as King of Finland instead of his elder twin Prince Philipp of Hesse (1896–1980), apparently because Wolfgang was with his parents in 1918 and ready to travel to Finland (where rumoredly a wedding to a Finnish lady already was in the preparation for the coming Crown Prince). Philip was in the military and incommunicado at the time.

• 1981 - King Randor and Queen Marlena of Eternia - Prince Adam, aka He-Man, and Princess Adora, aka She-Ra, Princess of Power
  

And there are lots of other Earls, Counts, children of Princes and Princesses (not Kings and Queens), and other people who married into royalty and whatnot, but who cares, y'know? Not me.

Source: Yahoo! Answers, Wikipedia

The More You Know: And, well, there was the Man in the Iron Mask - who was a real person, btw, not just a Dumbass character.

Also, here are some other famous twins. I blog about them kind of a lot. I think they are just fascinating.