Using a sociocultural theoretical lens, this study examines the nature of student interactions in a dual immersion school to analyze affordances for bilingual language learning, language exchange, and co-construction of language expertise. This article focuses on data from audio- and video-recorded interactions of fifth-grade students engaged in joint writing activities (in Spanish and English). A qualitative analysis of discourse found that students seized opportunities to use two languages simultaneously, which multiplied opportunities for metalinguistic analysis and bridged understanding across interlocutors. Findings suggest that language learning affordances could be fostered in linguistically diverse classrooms by allowing interplay between languages and by creating activities that encourage learners to co-construct text. This study contributes to the expansion and reconceptualization of the field of language education research by attending to bilingual language learners, or first language/second language users, whose reciprocal language learning experiences show how concepts from the fields of second language acquisition and bilingualism are necessarily linked. This study also contributes to language learning research using a sociocultural perspective by revealing the ways that two languages can simultaneously become mediational tools and objects of analyses within bilingual interactional spaces.