Sometimes it depends on the day, on the situation; when someone has more than just English and Spanish going in their heads. Things like bills, university entrance exams etc. The brain is in "overload" in a sense.
Other times, one language gets in the way of the other. Say, someone seeing some initialisms (siglas) with "do nordeste" and seeing a sign with DO NE (do Nordeste) in it, you could read the verb DONE where it doesn´t exist!
It´s something similar to when you see something and all of sudden you have to look at it again, because somehow you have to check it out, your "subconscious" says to you that something wasn´t to be there! That happens, more so when you are with your head in a thing/place and actually in a different place or doing something else.
And, obviously, the more you practice the more you get used to do two or more things, when you are in the process of learning to drive you are highly self-concious. You are aware of each and every movement you make, then after learning, you do it almost automatically, you can even talk to others! LOL. http://www.omniglot.com/language/articl ... guages.htm