However influential the interdependence hypothesis has become in bilingual research, it still lacks full empirical support. This longitudinal study explores the parallels in the biliteracy development (L1 Spanish and L2 English) of 20 students in a European immersion programme (i.e. CLIL) over a two-year period. A bilingual learner corpus of history narratives, based on history curriculum content, was collected during ninth and tenth grade. These essays were processed with MultiAzterTest, a state-of-the-art language analysis tool, and a Pearson correlation analysis was conducted to determine if any dimensions evolved in unison in both languages. The results show that some dimensions -- length measures, nominalisation, subordination and lexical development -- evolve in a similar fashion, thus supporting the interdependence and the common underlying proficiency hypotheses. Additionally, the results of a mixed-model analysis confirm that the fixed effect of time and language on such progress is significant, unlike the random effects introduced by the students.