Brad:

Once you (as a US citizen) enter the Schengen visa area, you can travel freely between all of the Schengen countries.

Occasionally, you might encounter a "pop-up" border control, but those kind of things are primarily for customs (not immigration) control of European citizens, for example, to catch folks bringing marijuana into Germany from Netherlands, or meat/milk into Switzerland from Germany. There's not much motivation to smuggle anything between Spain & Portugal, so it is very unlikely you will encounter a pop-up customs control on that border.

I'm a Canadian citizen & ride extensively through Europe several times a year. I've never encountered an immigration control from one Schengen country into another, but maybe once every 2 or 3 years I might encounter a customs control.

Unless you pay attention to signs at the side of the road, you won't even know that you have crossed from Portugal into Spain or vice-versa.

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As for "tips on where to go", if you are in the north of Portugal there is some great riding in the Galicia area, which is the part of Spain that is directly north of Portugal. Superb mountains, great roads.

There is really nothing worth seeing in Spain if you are south of about halfway between Porto and Lisbon. South of that point, western Spain is very lightly populated, towns are few and far between, and the area is mostly near-desert - think of what it is like riding in rural Nevada, USA.

But in the north - in fact, anywhere in Spain that is north of Portugal, or anywhere in Spain that is both west and north of Porto - the riding is superb.

I've done a lot of riding in Portugal over the past 20 years, and if this is your first trip, my suggestion to you is don't go south of Lisbon. All the great scenery, all the history, all the twisty roads & good riding are north of Lisbon, the farther north, the better. It won't be too cool in October to ride in the north, but do bring layered clothing, because there are some great mountain areas in Galicia and it can get cool at the higher altitudes.

Michael