I'm not so sure any more...
My classroom now is one way dual language. All students, except one outlier, are native spanish speakers. I'm the only native english speaker. We are all bilingual, some more than others, so we go between languages on a whim. Students translate when meaning gets lost in a language, but we are all comfortable in each language. This bilingual dynamic changes when we are in an English setting. When english speaking volunteers come into the classroom or when we go on an "english field trip" my students only speak English.... They need to communicate with people who don't speak two languages.
So what would happen if you throw monolingual English speakers into a classroom of bilingual Spanish speakers? The bilingual student will speak english, naturally, so the monolingual student can understand....Spanish isn't in the picture, especially in social settings.
....Only the Spanish L1 (native Spanish speakers) students were directly practicing their L2 (English) with their peers. In this, they supported Valdés’ (1997) concerns that mixing monolingual English L1 speakers with bilingual Spanish L1 speakers in two-way immersion will inevitably lead to students speaking English with one another to accommodate the monolingual speakers’ needs, and that only the English speakers will benefit from the mixed language enrollment that two-way offers.(Ballinger, S. & Lyster, R. 2011)
Maybe if the school was in a vacuum, and they all started in Kinder, and stayed till 5th with no one transferring in halfway, my perfectly balanced two-way school would work....but somehow i think this environment doesn't exist, and until then, who's benefiting? One student is gaining a language, while another student is losing one.