Aims and Objectives/Purpose/Research Questions: We investigate the robustness of cross-language phonetic correspondences in two bilingual communities over time, focusing on whether corresponding sounds (e.g. Mandarin /s/ and Korean /s’/) remain coupled in the face of language change, or whether the categories diverge over time in younger, more proficient bilinguals. Design/Methodology/Approach: We quantify the extent of assimilation versus independence of categories across languages by comparing bilinguals’ production of place of articulation and laryngeal contrasts in Mandarin and Korean sibilants. Distinct language-internal changes were expected on each dimension. Data and analysis: 107 speakers varying in age (aged 19–83), gender, and dialect participated in the study. Acoustic measurements (center of gravity of frication, voice onset time) and statistical analyses were performed on a total of ~11,000 tokens. Findings/Conclusions: The extent of cross-language independence differed on the two dimensions. Corresponding segments across the two languages remained tightly coupled in terms of place of articulation, even in the face of change; on the other hand, a language-internal change in the Korean laryngeal contrast left corresponding Mandarin segments unaffected, resulting in divergence of originally corresponding categories. We also found unpredicted changes on each dimension, and these changes progressed concurrently in the two languages. Originality: The study of correspondences in the context of independent sound change provides a unique perspective from which to evaluate the robustness of cross-language interaction, and the parallel analysis of two separate dimensions in two communities adds to the generalizability of results. Significance/Implications: Most changes occurred concurrently in the two languages, suggesting that similar phonetic categories across languages can remain tightly coupled, even in highly proficient bilinguals where phonetic independence is expected. However, one of the primary expected changes (voice onset time merger in Korean) did not affect corresponding segments in Mandarin, indicating that the extent of cross-language independence in phonetic correspondences may differ even within the same population. We discuss potential reasons for the different results.