That would rather defeat the purpose of the study... if neither language is really foreign to you how could we look at the difference using a foreign language makes?
Ok, you're right. I was trying to compensate the fact that the language is learned in a classroom as opposed to in real life. I guess I had the intuition that bilingual people still have a language that is "more native" than the other, but I don't know if that is true.
However, I don't understand the downvote?