I know François Hollande, as the president of France, is one of the chiefs of Andorra. The other one, I think, is the bishop of Andorra La Vella. Why he instead of the Spanish king, chief of the Spanish government, the other country with strong cultural and political connections to Andorra?
Andorra was founded in 1278 through a treaty between the Bishop of Urgell and the Count of Foix who became co-princes.
In 1479, Francis Phoebus of Foix became King of Navarre and in 1572 Henry II of Foix (and Henry III of Navarre) became King Henry IV of France. In 1607 the position was formally passed to the French Crown. France renounced the position during the First Republic, but Napoleon I reclaimed it as emperor. When Napoleon III became President of the Second Republic, he kept it, and it has since been held by all French Heads of State (including Pétain during WWII).
Urgell, meanwhile, was also a county at the time of the original treaty (under Ermengol X) and in 1413 it too got absorbed, into the County of Barcelona as part of the Principality of Catalonia and the Crown of Aragon. However, the Diocese of Urgell was unaffected and so its bishop remained co-prince.