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The interactions of multiple flames are often encountered in real-world fire and industrial burners. The distance between two jets has a strong influence on the instability mode, oscillation frequency, and mean height of flickering diffusion flames, which is experimentally studied and analyzed in this paper. Five different types of instability modes are identified as the separation distance between two nozzles is increased. When the nozzle separation distance is smaller, the flame mode is similar to that of a single-nozzle flame. In this case, the flame can switch between the merged sinuous mode and the merged varicose mode due to external disturbances on the flame. As the nozzle separation distance increases, the probability of mode switching from merged varicose to merged sinuous decreases. As the nozzle separation distance increases further, the flame mode translates into symmetric sinuous mode, alternated sinuous or independent mode. In addition, the flame height and oscillation frequency of a dual-nozzle flame have their own characteristics, which are different from that of the single-nozzle flame. The increase of the nozzle separation distance, leads to a decrease and then an increase of the flame mean height. It is interesting that the flame mean height of the alternated sinuous flame is the lowest, even lower than that of the single-nozzle flame. The oscillation frequency of the symmetrical sinuous flame is lower than that of the single-nozzle flame, and the frequency of alternated sinuous flame is higher than that of the single-nozzle flame.