This study investigates cross-linguistic influence (CLI) and the development of cross-linguistic awareness in a beginner-level L3 Korean class composed of advanced L2 Japanese learners at a UK university. Drawing on classroom observation, learner reflective essays, and teaching practice, the study examines both facilitative and non-facilitative effects of L2 Japanese on L3 Korean learning. Learners anticipated considerable benefits from the typological proximity of Japanese and Korean, and cross-linguistic comparison was found to reduce anxiety, support comprehension of grammatical structure, and facilitate vocabulary acquisition. However, learners also experienced non-facilitative effects, including interference at the phonological and orthographic levels, full transfer attempts in discoursal aspects, and cognitive overload from managing two similar languages simultaneously. Over time, learners developed a deliberate separation strategy, gradually shifting from using Japanese as a primary framework to treating it as a contextual reference. Notably, cross-linguistic influence extended beyond linguistic structure: learners also transferred metacognitive strategies and learning techniques developed through Japanese study, repurposing them productively in the Korean context. Pedagogical implications for teaching multilingual learners with closely related prior languages are considered.