the answer is: Programs that combine English and Spanish instruction, with sometimes a half a day for each
People's ability to use language is really influenced by how often they implement the language in their life.
So, for most people, when they are trying to increase their skills in two different languages, their fluency in one of the language will remain stagnant or decreased if they focus too much on implementing the other language.
Because of this, the research found that the most effective way to do it is by combining both language and condition yourself to use it separately on different time throughout the day.