A growing interest in Two-Way Bilingual Immersion (TWBI) programs has led to increased attention to bilingualism, biliteracy, and biculturalism. This article describes the writing development in Spanish and English for 49 kindergarten students in a 50/50 Two-Way Bilingual Immersion program. Over the course of an academic year, the authors collected writing samples to analyze evidence of cross-linguistic resource sharing using a grounded theoretical approach to compare and contrast writing samples to determine patterns of cross-linguistic resource sharing in English and Spanish. The authors identified four patterns: phonological, syntactic, lexical, and metalinguistic awareness. Findings indicated that emergent writers applied similar strategies as older bilingual students, including lexical level code-switching, applied phonological rules of L1 to their respective L2s, and used experiential and content knowledge to write in their second language. These findings have instructional implications for both English Learners and native English speakers as well as for learning from students for program improvement.