One last name in Passport, Two last names in plane ticket

Question

I bought a ticket to travel to Spain from Los Angeles through Iberia, and when I entered my name, without thinking I put my first name, paternal last name and maternal last name, and I realized my mistake until I received the ticket confirmation; they put my first name, a space and my two last names together.
The problem is that I am going to fly with an American passport, which only has the first last name.

I have spoken to Iberia several times and they always tell me that there is no problem, that as long as the passport shows the first name and the first last name, that's fine, the second last name is extra and they will not take it into account and they will give me my boarding pass without problem.
What worries me mostly are the TSA agents at the airport, what would they do if the ticket has the second last name attached to my first last name, when my American passport does not. As an example, My name is AAA BBB CCC, my passport only shows AAA BBB and my plane ticket shows AAA BBBCCC.

I also have a Mexican passport that shows my two last names, but it has been expired for several years, since I have not used it in a long time; Is it worth taking it with me just in case? And if it's expired, will they take it into account, just to show them that my maternal last name does exist?

Has anyone had an experience like this and know what could happen?

Answer 1

I also have a Mexican passport that shows my two last names, but it has been expired for several years, since I have not used it in a long time; Is it worth taking it with me just in case?

Yes. It might come in handy and it's neither big nor heavy.

And if it's expired, will they take it into account, just to show them that my maternal last name does exist?

Since you're not going to be using it as your primary means of identification (much less sole means of identification), the fact that it's expired probably won't matter terribly much.

Has anyone had an experience like this and know what could happen?

My experience is that discrepancies in given names are tolerated. I can imagine that surnames would be treated more strictly, but with the huge number of people in the world who have Spanish names, this surely must happen fairly frequently. If it were preventing people from clearing TSA, we'd have heard about that before now.