This study investigated how students learning English and students learning Spanish activated multilingual repertoires as they participated in one high school program that aimed to promote reciprocal learning and teaching of multilingual literacy practices. Grounded in sociocultural theory, we examined how students drew upon Spanish, English, and translanguaging as cultural and cognitive tools to mediate learning in a Third Space. Data collection included participant observations in 40 sessions, student writing, interviews, and audio/video recordings of peer interactions as they engaged in composing and revising of text together. Using interactional ethnography and microgenetic analysis, we analyzed mediation of learning opportunities across and between languages and found evidence of students co-constructing knowledge and expanding multilingual repertoires. Findings contribute to second language acquisition research by revealing fluid and reciprocal affordances for language learning during interactions among linguistically diverse peers as they draw upon translanguaging practices. By shedding light on an alternative educational context that mobilizes young people's diverse funds of knowledge, the findings have implications for educational practices that support equity for culturally and linguistically diverse students.