This paper presents the findings of a bidirectional study of the acquisition of number marking by adult native-language (L1) Korean speakers acquiring Indonesian as a second language (L2) and by adult L1 speakers of Indonesian acquiring L2 Korean. We compare the learning vs. unlearning of restrictions on pluralization, and further consider whether features embedded more deeply in a feature co-occurrence hierarchy are more difficult to acquire (following Hwang & Lardiere, 2013). For the L2 Korean study, 70 L1 Indonesian speakers and 40 native Korean-speaking controls participated in three tasks (in Korean): a sentence completion task, a grammaticality judgment task, and a multiple choice task; for the L2 Indonesian study, 61 L1 Korean speakers and 39 native Indonesian-speaking controls were presented with the same three tasks (in Indonesian). Findings from both studies indicated that performance on number marking gradually improved as L2 proficiency increased. However, whereas the more advanced-proficiency L2 Indonesian learners did not significantly differ from the Indonesian native-speaker controls, the performance of even the most advanced L2 Korean learner group was significantly lower than that of the Korean native-speaker control group. More specifically, the more deeply-embedded [±human] feature distinction required for nativelike knowledge of pluralization in L2 Korean was not easily acquired, in line with previous findings by Hwang and Lardiere (2013) for L1 English learners of L2 Korean and despite closer similarities in number marking between Korean and Indonesian than between Korean and English.