Castille/Aragon doing that is going to have a really bad surprise.
While the OTL Iberian Union was realized with Spain being clearly hegemonic in Europe and Americas, in the 1490's not only Portugal mastered Atlantic deep enough for disallowing Castille the possibility to follow its steps in Africa.
Furthermore, if Castille would have attempted something, it would be to prevent such union, not hastening it. Without the death of Alfonso of Portugal, it's Portugal that would have a strong claim on Castille/Aragon, not the reverse, and Castillan nobility clearly didn't wanted such thing to happen.
Critically giving that Aviz dynasty already tried to get Castille, during Alfonso V's reign. If something it points that Castillans weren't interested on an union, and that Portugal was pretty much able to hold face to Castille in war.
Assuming that every Castillan/Aragonese noble, from king to knights, all drink White Spirit and dissolve their brain to act on Portugal, Leroy Jenkins way, you'd end with a switch of alliances, beneficing Portugal.
Not only the old Anglo-Portuguese alliance, but probably giving a good opportunity for Charles VIII to turn against Aragonese interests in Italy and in Spain.