King Edward I of England, Lord of Ireland married Infanta Isabella of Castile, daughter of Fernando III of Castile and Joan, Countess of Ponthieu. Edward I’s second marriage was also to the daughter of a king. He married Margaret of France the daughter of Philippe III of France and Maria of Brabant.
King Edward II of England, Lord of Ireland married Isabella of France Her parents were King Philippe IV of France and Queen Joan I of Navarre.
The second spouse of Richard II of England was the child bride Isabella of Valois and her parents were Charles VI of France and Isabeau of Bavaria.
King Henry VI of England, Lord of Ireland married Margaret of Anjou the second eldest daughter of King René of Naples, and Isabella, Duchess of Lorraine.
On May 1, 1625 King Charles I of England, Scotland and Ireland married the fifteen-year-old French princess Henrietta Maria de Bourbon the youngest daughter of Henri IV of France and Navarre and his second wife, Marie de' Medici
On May 21, 1661 King Charles II of England, Scotland and Ireland married Catherine of Braganza, daughter of King John IV of Portugal and Luisa de Guzmán.
Felipe IV of Spain married as his wife Elisabeth of France, the daughter of Henri IV of France and his second wife, Marie de' Medici.
In 1615 King Louis XIII of France and Navarre married Anne of Austria the eldest daughter of King Philip III of Spain and his wife Margaret of Austria. She held the titles of Infanta of Spain and of Portugal (since her father was king of Portugal as well as Spain) and Archduchess of Austria.
In 1660 King Louis XIV of France and Navarre married Infanta María Theresa of Spain the daughter of Felipe IV of Spain his wife Elisabeth of France, herself the daughter of Henri IV of France and his second wife, Marie de' Medici.
Years ago, I posted a list of royal women who, at some time or other in their lives, were both daughters and wives of kings. The list must be an extensive one, if you're going as far back in history as possible; as it was, I restricted the discussion to only the past two centuries.
That being said, I made the list broad, in that the accessions of the king fathers and king husbands could have been at any time. Queen Ingrid of Denmark (born a princess of Sweden), for instance, was the consort of King Frederick IX, who ascended the Danish throne in 1947, twelve years after their marriage and three years before her own father (King Gustaf VI Adolf) ascended the Swedish throne. So although he was alive and kicking at his daughter's wedding, the latter obviously was not an enthroned giving away a bride to another enthroned king.
Such a thing is indeed a rare occurrence, but it has been known to happen; and when it does, it is undoubtedly a grand event: Ingrid's own youngest daughter found herself in such a situation later on. Princess Anne-Marie of Denmark was a daughter of an enthroned king, when in 1964 she was given away in marriage to another enthroned king, Constantine II of the Hellenes. I believe this was the last time in European royal history that such a thing happened.
The only other example I can think of in the last century, off the top of my head, is the marriage in 1922 of King Alexander I Karageorgevich of Yugoslavia and Princess Marie ("Mignon") of Romania, second daughter of King Ferdinand (who had succeeded his uncle on the throne in 1914).
The statistical odds of this happening are not great; so I was wondering about other examples, going further back in history. There is nothing unusual, for instance, about an enthroned king (whether a bachelor of widower) getting married. But for his bride to specifically be a daughter of another enthroned king is another matter (she may be the daughter of a deceased king, or of a reigning monarch who was not a king, or only a granddaughter of a monarch). Or a king might give his daughter away at her wedding to a crown prince, who only later becomes king himself.
Indeed, given Anne-Marie's youth, the terms of the engagement were that she must first finish school and obtain her legal majority, before being permitted to marry (King Frederik IX of Denmark initially withheld his consent to the union). She and her future husband originally planned for their wedding to take place in January, 1965 -- two years after the two got engaged. As it was, the death in March 1964 of his father, King Paul of the Hellenes, sped things up. The new King Constantine II married his bride later on that year, in September (a couple of weeks after she celebrated her 18th birthday).
Come to think of it, it's a rare occurrence even for children of two reigning monarchs to marry. The last time this happened in European royal history was in 1982, when Prince Nikolaos of Liechtenstein (son of Prince Franz Joseph) married Princess Margaretha of Luxembourg (daughter of Grand Duke Jean). Neither sovereign was a king, and the groom was not even heir to the throne; but it had to have been a grand occasion, nonetheless.
Can anyone name other examples?