The Network Time Protocol (NTP) is a server-client-based time synchronization protocol that transmits time information over a network, and is used in various applications on the Internet. Especially, the NTP Pool Project is designed to connect NTP servers that provide accurate time to millions of clients, and balance the load on the NTP servers using the NTP pool. The NTP pool operates the NTP pool monitor system to evaluate the time accuracy and availability of the NTP servers registered with the NTP pool. There are currently two operating environments for monitoring systems in practice: one is a single monitoring system, which is officially operating, and the other is a multiple-monitoring system, which is now under beta testing. In this study, we investigate the NTP pool monitoring system based on multiple monitoring stations, which is expected to be deployed soon in the real world. We then discuss possible threats and their security implications when the current single monitoring system extends to the multiple-monitoring system.