At first I was thinking that a Portuguese-Castillian union ending with a single state would mean the Portuguese would monopolize both the new World and Asia
But then again Columbus asked Portuguese support before the Spaniards, so if we have a Portuguese-Castillian union, another country would find the new world
Christopher Columbus's brother Bartholomew asked support to the French, which in OTL didn't give and I think it wouldn't change ITL
The probable candidate to find the Americas IMHO is one of the Italian city states
Venice, the once only maritime power in the Mediterranean is slowly loosing it's grip there from competition with the Ottomans
Genoa, which is notably the Columbus brothers' hometown
Both city states rely heavily on trade, and they suffer heavily with the discovery of a sea route to the spice islands
The Columbus brothers did ask for support to both city states, which was denied because the two city states still control the spice trade
But I wouldn't say there would be an Italian new world as the problem with Italian city states, there wouldn't be enough colonists and soldiers to successfully subdue a whole continent
And as merchant states looking for goods, unless they arrive at Mexico instantly and find the gold there, they would view this as an invaluable piece of land and just leave it
So even if the Americas did get discovered, they won't get colonized as fast as they did with the Spanish
But if they do get discovered, the natives will have enough time to adapt with European diseases, making great civilizations like the Aztecs and Incas much harder to beat