In dense radio frequency identification (RFID) systems, reader-to-reader interference may appear among readers operating nearby. When the anticollision algorithm is Frame Slotted Aloha (FSA), a usual solution is to multiplex the frames of interfering readers by means of a central controller. The scenario studied considers RFID tags attached to objects carried by conveyor belts or vehicles so that each tag crosses, in constant motion, one reader's interrogation region. The problem addressed consists of determining the length of the multiplexed FSA frames such that each object is successfully identified, with high probability, before exiting its interrogation region. In this paper, we develop a mathematical model, based on dynamic system theory, allowing us to obtain the configurations attaining the desired reliability objective, to show that some configurations present multiple equilibrium points and should be avoided, and to define the operating boundaries of the system. The model can also incorporate fading and capture effects.