Most studies on bilingual children's metalinguistic awareness assess metalinguistic awareness using monolingual tasks. This may not reflect how a bilingual's languages dynamically interact with each other in creating metalinguistic representations. We tested 33 Greek-Italian bilingual children (8-11 years) for metalinguistic awareness using acceptability-rating tasks in which they had to judge and explain grammatical errors. The tasks were in monolingual and bilingual modes in order to show how far metalinguistic awareness in Italian benefited from the activation of Greek. The participants exhibited better metalinguistic awareness abilities in Italian in the bilingual acceptability-rating task in which Greek was activated. The benefits of the bilingual mode were visible in the judgment and explanation of errors and were modulated by syntactic processing abilities in Italian, length of exposure to Italian, type of structure, and age. The results show that metalinguistic awareness can be shared across languages. We discuss the pedagogical implications of our findings.