This study investigates whether syntactic unification occurs during online L2 sentence comprehension using time-frequency analysis. We measured the oscillatory power changes in native English speakers and L1-Cantonese L2-English speakers while they were reading well-formed English sentences, syntactically intact nonsense sentences, and random word lists. Additionally, we conducted traditional ERP analyses to test L2 speakers' sensitivity to NP-internal number (dis)agreement. The results show that low-beta power significantly increased in the L2 group when reading not only well-formed sentences but also nonsense sentences, replicating the pattern found in the L1 group. This suggests that syntactic unification occurs in L2 comprehension as reliably as in L1 comprehension. However, L2 speakers did not show increased positivity for NP-internal number disagreement, indicating that they have not developed native-like sensitivity to this syntactic error. The implications of these time-frequency and ERP data for L2 sentence processing and syntactic development are discussed.