In linguistically diverse contexts, language repertoires include various literacy skills in multiple languages across different modes of representation (multiliteracies), where multilingual writing can be conceptualized as a synthesized competence that includes all languages in a person's repertoire and is continuously evolving. We respond to the call to operationalize existing theory on multilingual writing for empirical testing by conducting first- and second-order confirmatory factor analyses that explore whether and how students' writing proficiency in different languages can be modelled as an integrated construct, to analyze multilingual writing's dimensionality. We draw on a corpus of multilingual writing in secondary students (n = 965) in three different languages: the majority language (German), the heritage languages of migrants in Germany (Russian or Turkish), and the first foreign language (English). Based on competence data of a substantial sample of multilingual adolescents, the results provide an empirical approach to modelling multilingual writing competence in complex multilingual repertoires.