Babies who exposed to two languages start with

The advantages of growing up in a bilingual home can start as early as six months of age, according to new research led by York University's Faculty of Health. In the study, infants who are exposed to more than one language show better attentional control than infants who are exposed to only one language.

the one they use most

Explanation:

Because when you teach a child two languages the one that the parents use most is more likely to be used often.  for example if you speak spanish most of the time then the child will learn that quicker.