@Antonio the Komnenoi I should probably warn you that some Portuguese members of alternatehistory.com (especially Lusitania) tend to get a little bit worked up every time something like this is discussed. Please do not feel offended if the discussion ends up getting a little... spirited.
Anyway, I'll try my best to explain why this is impossible, as I realize it may not be clear to non-portuguese people (even other lusophones). The reason why Portugal and Spain could never unify is not so much because of cultural differences (I recognize that these are relatively few, and probably comparable to the differences between Spain's own several autonomous communities), but more because of the way Portuguese national consciousness developed historically. The Kingdom Portugal was born in the middle ages out of a rebellion agaisnt the Spanish-precurssor Kingdom of Leon. It is important to note that this rebellion did not happen due to any cultural differences between Portugal and the rest of Leon but more because Portuguese noblemen (or, rather, the noblemen in the region between Porto and Gaia- which was the original extent of Portuguese domains) felt that Leonese crown was not protecting their interests. As a result, Portuguese culture and national identity, which only developed after the independence, have always placed themselves in direct opposition to Leonese, and later Castillan and Spanish culture and identity.
Not being Spanish is a core aspect of being Portuguese. Resistance to assimilation by Spain is a sentiment that is too deeply ingrained into the Portuguese mindset for any sort of unifcation to be possible. The fiasco of the original Iberian Union (which, by the way, happaned during a time of immense Spanish prosperity, and still, it was short-lived and never got to a point in which it could be said that the two countries were effectively one) served to greatly reinforce these notions.
As such, I think it's very safe to say that any sort of iberism during this time is doomed to failure right from the start.