The catastrophic tunnel fire accidents are usually accompanied with multiple fires burning. This work experimentally studied the temperature-determined flame extension below the ceiling induced by two unequal square gaseous fires in a 1:10 scale tunnel with natural ventilation. The two fires were parallel and both placed along the longitudinal centerline of tunnel. The heat release rate (HRR) of each burner and the burner spacing were changed. The measured ceiling gas temperatures were used to obtain the flame length and tilt angle. Results showed that for small fire groups, the spacing has a small effect on the flame length. For large fire groups, the variation law of flame length of the small fire with spacing is different from that of the large fire. The flame tilt angle is analyzed to illustrate the variation of flame length. A normalized parameter α representing the spacing effect and the equivalent burner size concerning the weight of the burner perimeter is introduced to modify the HRR of each burner. Then a new model for flame length of two asymmetric fires with large HRRs in a tunnel is developed. The model for flame length with small HRRs is proposed based on the single fire assumption. Regarding the two merging fires as a virtual fire source, the model for flame length of virtual fire is established. Then the critical merging criterion is proposed based on the normalized spacing and HRR. Regarding the two fires with zero spacing as a single circular fire source, the point source model is adopted to calculate the overall flame outward radiation, and the personal safety distances under different HRRs in tunnel are further determined. The present results can be well used to predict the flame lengths of two unequal fires in tunnel and provide some useful references for personal evacuation, fire rescue and risk assessment in tunnels.