Open-source software (OSS) has gained prominence in both academia and industry. Despite the fundamental role of forking in OSS, which initiates an independent line of development from an existing project, the coordination mechanisms between cohabiting OSS communities post-forking remain underexplored. To address this gap, we study the Bitcoin-Bitcoin Cash fork, drawing on extensive quantitative and qualitative data from GitHub repositories. Our mixed-method analysis, which combines exploratory data analysis, code comparison, keyword matching, and inductive coding, leads to a set of propositions suggesting that: (1) coordination via shared developers is limited; (2) knowledge flows from parent to fork as functional code reuse through backporting; (3) knowledge flows from fork to parent primarily consist of non-functional code changes and bug reports; and (4) governance mechanisms, such as licenses and copyrights, regulate these flows. These insights deepen the understanding of coordination between OSS communities and contribute to the broader literature on OSS.