Writing, as a highly complex strategic literacy skill alongside reading, is an essential prerequisite for learning and determines a student's educational success. In diverse contexts, a student's linguistic repertoire may involve multiple languages, which may serve as mutual resources in his or her multilingual writing skill development. Drawing on the data from a German panel study, "Multilingual Development: A Longitudinal Perspective", the current research used the longitudinal writing competence data of 965 German-Russian and German-Turkish secondary students regarding their majority language (German), heritage language (Russian or Turkish), and foreign language (English). We applied longitudinal structural equation modeling to investigate within- and between-language effects in multilingual writing development over three waves of data collection. Accordingly, our study extends previous research on the interrelation of languages in multilingual writing development in two ways. First, we provide a more comprehensive analysis of migrant students' multilingual writing repertoires by simultaneously evaluating three languages in an integrative model of multilingual writing development. Second, we use longitudinal competence data to decompose covariance between languages to isolate the parts of the variances that truly predict changes within and between languages. This approach empirically tests the resources hypothesis more rigorously than extant evaluations. In summary, our findings indicate that language-specific writing skills may serve as mutual resources for developing multilingual writing proficiency.