<p>Multiple fires can occur simultaneously inside and outside the tunnel entrance and exhibit unique behaviors. This work carried out numerous experiments to investigate the burning and flame characteristics of double fires separated inside and outside the tunnel under natural ventilation, using square ethanol pool fires with 3 side lengths and 17 dimensionless fire separating distances. The tests with a single fire source were conducted for comparison. Numerical simulations were performed for the description of flow field by FDS. Results show that when a single fire is located near the tunnel entrance, the difference in pressure drop and airflow distribution on different sides is formed, resulting in the tilted flame. For the two fires, the unbalanced air entrainment of the inside and outside fires enhances the asymmetric distribution of induced airflows, resulting in evident asymmetric features of fire behaviors. The variation of the burning and flame characteristics differs as the dimensionless fire separating distance is larger or smaller than 2.5, which can be grouped into Regions I and II. The transition of the flame characteristics is also influenced by the fire size. Additionally, the global power function correlations of the dimensionless flame height of the inside and outside fires are established.</p>