Vuelta a Espana was a sping race for decades, the first autumn edition was held in 1995, and its original schedules were usually very close to Giro d'Italia. Little wonder, that the first Giro-Vuelta happened only in 1973, and the first rider to achieve it was Eddy Merkcx.
(Just for the record: Fausto Coppi managed the first Giro-Tour double in 1949, and Jacques Anquetil the first Vuelta-Tour double in 1963)
Also, Vuelta a Espana seemed to be like an ugly stepsibling in comparison with the other two grand tours. Some of the greatest legends of this sport, like Gino Bartali,never competed the race. (The first Vuelta a Espana was held in 1935, thus technically it would have been possible to participate.) Fausto Coppi rode Vuelta only once, and so did Eddy Merckx.
Merckx skipped the Tour de France 1973 to focus more on the other two grand tours. The schedule was tight with only five days between the end of Vuelta a Espana (13 May) and the start of Giro d'Italia (18 May). It's worth to take a look at the top of the general classification of this Vuelta: behind Merckx, Luis Ocaña became second, and Bernard Thévenet third. During the Giro d'Italia, Merckx's strongest rivals were Felice Gimondi (2nd in the GC) and Giovanni Battaglin (3rd).
Although there are riders every year, who participate both Giro d'Italia and Vuelta a Espana during the same season and are considered favourites of both races, besides Merckx, there are only two other riders, who managed to win the Italian and the Spanish grand tour in the same season, Giovanni Battaglin and Alberto Contador.
Short list of Giro-Vuelta double winners
Eddy Merckx (1973)
Giovanni Battaglin (1981)
Alberto Contador (2008)