AFAIK all early means to get a unified Portugal-Castille will have the Kings of Portugal in the driver's seat. This whole shebang is effectively running out of Lisbon early on and gonna have a strong Portuguese presence in colonial ventures, to the point that someone akin to Colombus might sail for Portugal instead of Castille. Heck, even the treaty that preceded the Treaty of Tordesillas, the Treaty of Alcacovas, is likely to have ended with a treaty that heavily favors Portugal. Maybe even to the point, that the Canaries were seized from Castille and Portugal established a monopoly on Morrocco and the Atlantic.
If Portugal's leading the charge on colonialism and the semi-subservient Crown of Castille is legally blocked out of the Atlantic then there's a very strong possibility that all colonialism by this ATL Spain is managed by Portugal, but leverages the manpower of all of its constituent kingdoms. It provides a means to either equalize the political weight of each component realm, or to centralize the realm around the King of Spain if colonization rights are leveraged to extract concessions that can break down barriers between the realms that make up Spain. Maybe a renegotiation of the ATL Treaty of Alcacovas that gave Portugal a monopoly is renegotiated internally to establish a state apparatus that overrules both kingdoms, such as a shared Spanish cortes?
Assuming it's still Colombus and his voyages are similar to OTL, then there's a good chance that Caribbean exploration and early colonization go as OTL. Where things get interesting is that Portuguese interests will also have to be catered to, and that likely means a very early settlement of Newfoundland for its fisheries. With no colonial conflicts, somewhere like Montevideo is likely to be colonized and populated sooner than OTL due to zero ambiguity on who controls the Rio de La Plata. The OTL conquests of the Aztec and Inca are likely to be butterflied or look way different than they did, the Inca especially which depended on near-perfect timing by the conquistadors.
And that brings us to the final question mark, Morrocco. With a unified Portugal-Castille I honestly think that Morroco's days are numbered. With no more bickering or political considerations to hamper plans, a unified invasion of the area fueled by New World wealth and no Holy Roman or Italian commitments seems imminent and this is likely to make use of the conquistadors that OTL went to the New World after the conquest of Granada. I guess on the bright side, the expulsions of Muslims and Jews from OTL are less likely to occur, though by no means unlikely. Portugal was a more tolerant state than Castille ever was. This is also likely to butterfly much of the conquests of OTL with the manpower that originally went towards the New World's conquistadors being refocused on Morocco.
The final thing worth bringing up is that the language of the people is likely to see significant shifts(IMO) given enough time. Ships and armies are going to draw from both Portugal and Castille and it's likely that a pidgin somewhere between the two eventually develops such that it's a smooth process to bring on more manpower quickly. Think of it the same way you might think of Sabir, except that this language is gonna have some ramifications on the identity of the state and the slang of the people that may eventually make its way into proper vocabulary. I don't think that we'll get a situation where one language supersedes the other or that Iberian-Sabir is gonna be the national language of a unified Spain in the modern-day, but there's potential there for an even higher degree of mutual intelligibility than OTL if pronunciations are warped. My understanding is that the Portuguese seem to be able to grasp what a Spaniard is saying if they talk slow, but Spaniards claim they can't understand anything Portuguese speakers are saying. Lines up with my personal anecdotes at any rate