Queen Mary I of Scots married the future King François II of France the son of King Henri II of France and Catherine de' Medici.
Mary’s son, King James VI-I of England, Scotland and Ireland married Anne of Denmark the daughter of King Frederik II of Denmark and Norway and Sophie of Mecklenburg-Güstrow.
Henry VII of England married Elizabeth of York the daughter of King Edward IV of England and Elizabeth Woodville
When Queen Mary I of England married the future King Felipe II of Spain, his father, Holy Roman Emperor Charles V (Carlos I of Spain) ceded the crown of Naples, as well as his claim to the Kingdom of Jerusalem, to him.
Felipe II’s first wife was Maria Manuela, Princess of Portugal second child of King John III of Portugal and his wife Catherine of Austria.
Felipe II’s third wife was Elisabeth of Valois the eldest daughter of King Henri II of France and Catherine de' Medici. His fourth wife was his niece Anne of Spain eldest daughter of Maximilian II, Holy Roman Emperor, and Maria of Spain.
Charles V, Holy Roman Emperor and King of Spain married Isabella of Portugal second child and eldest daughter of King Manuel I of Portugal and his second wife Maria of Aragon.
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?