Radiation portal monitors (RPMs) are operated in various places such as airports, ports, and nuclear facilities to detect radioactive materials illegally possessed by pedestrians. However, most commercial RPMs can measure only gamma-rays; hence, separate neutron detectors need to be applied when neutron measurement is required. Radioactive materials including special nuclear materials (SNMs) have a high gamma-ray emission rate, but they can be concealed because gamma-rays are easily shielded. By contrast, although SNMs have a low neutron emission rate, it is difficult to shield them due to secondary radiation emission when neutrons are captured. Additionally, neutron-emitting materials can be easily detected due to the low background level of neutrons. So, measurement of both gamma-rays and neutrons is beneficial for improved detection probability of radioactive materials (i.e., SNMs). In the present study, by Monte Carlo (MC) simulations, we designed a dual-particle detection system incorporating NaIL scintillators for application as a pedestrian RPM that can detect neutrons and gamma-rays simultaneously. The simulation model was verified by experiments with a 2 inch × 2 inch cylindrical NaIL detector using gamma-ray and neutron calibration sources. The designed system is expected to offer improved detection performance over that of a commercial system, even with its smaller dimensions.