This study examines the online writing practices of adolescent emergent bilinguals through the mediating lenses of dialogic interaction and intertextuality. Using a multimodal discourse analysis approach, the study traces how three students develop online academic texts through intertextual moves that traverse modal boundaries. The analysis reveals how cross-modal, dialogic interactions allow emergent bilingual students to pool linguistic resources, background knowledge, and writing expertise. Such collaborative interactions ultimately serve to support their academic writing. Findings suggest the importance of developing research methodologies and literacy pedagogies that bridge boundaries across social actors, spaces, modes, and languages.