The linguistic interdependence hypothesis posits the existence of language features common to different languages. This set of characteristics, known as Common Underlying Proficiency (CUP), is a powerful facilitating agent in second language acquisition. Fluid intelligence (Gf), on the other hand, is the construct that encompasses those cognitive resources devoted to general learning, and its involvement in second language acquisition is unproven. The aim of this study is to determine the direct and interactive effect of L1 knowledge and Gf on second language acquisition in language immersion learners across different linguistic domains. The study analyzed the proficiency of 131 Romanian students in Spanish and Catalan, the official languages of Catalonia. Mixed-effects regression models were used to analyze lexical, morphosyntactic, and orthographic aspects. The results were obtained using mixed-effects regression models, revealing a particularly noticeable interdependence effect in lexical, morphosyntactic, and orthographic aspects, with differences between Catalan and Spanish. Furthermore, Gf had an impact on the morphosyntactic component with similar intensity for both languages but did not moderate the interdependence effect. The study discusses the possible causes of these effects, as well as their psycho-pedagogical consequences.