Multilingualism is often associated with advantages for acquiring additional languages. Theoretical approaches explain these advantages by assuming a Common Underlying Proficiency or a Metalinguistic Awareness. At the State Europe School in Berlin, students from different language backgrounds receive instruction in German and a partner language according to two-way immersion (TWI). It is unclear how this bilingual instruction affects the acquisition of a third language. We examined the English proficiency of N = 656 TWI sixth-grade students and N = 739 mainstream students via a C-test. Multiple regression analyses revealed that TWI students exhibited higher English proficiency than mainstream students despite having received less English instruction. The results showed additional effects of German- and partner-language reading skills. The findings support the assumption of TWI programs that the use of two languages of instruction fosters third language acquisition.