The question of teaching someone two languages at once has some evidence on the side of people being able to do so. My son had two new students come in to his junior high class speaking no English – one Arabic and one French (and probably Lingala). By the end of the year, both were speaking English and Spanish b/c Spanish in that school is the survival language – in fact, he had to persuade the parents of the girl from Congo that she really needed to know Spanish, just for survival in this very rough school.
The other piece comes from my own experience learning and teaching several languages: people would ask me if I got them mixed up. No, I said, b/c they each have their own system, entirely separate. The only mix-up that might occur is when I’m trying to think of the word for, say, pillow, in Russian and the Spanish word pops up first in my mind and blocks the Russian.