In an outpatient department of general hospitals, several doctors practice simultaneously. While individual doctors have their own patient panel and work independently, they share common resources such as space, personnel and equipments. In such settings, designing an optimal scheme to manage patient flow, e.g. appointment scheduling, requires to consider patient flows for all doctors instead of focusing on a single doctor. This paper examines an appointment scheduling problem for an outpatient unit where multiple doctors practice independently yet sharing common resources. An ophthalmology department of a large-scale general hospital in Korea is modeled in discrete event simulation. Our experimental results show that under multiple-doctor and resource-sharing environment, collection of the seemingly optimal appointment rules for individual doctors does not lead to optimal performance for the system. It implies that altering a patient flow, especially modifying the scheduling rule, should consider the interdependence effects within the system.