Original: The treaty was ratified by Spain (at the time, the Crowns of Castile and Aragon), and by Portugal,. The other side of the world would be divided a few decades later by the Treaty of Zaragoza or Saragossa, signed on, which specified the antimeridian to the line of demarcation specified in the Treaty of Tordesillas.
Simplified: One from Castile and one from Aragon This treaty was signed at Tordesillas, hence the name of it.