In this work, we have carried out a parametric study on the interaction of two streamers in air using 2D numerical simulations based on an accurate numerical method which ensures a time-space error control of the solution. This study is a first step towards a quantitative analysis of streamer merging in real 3D conditions. We have shown that for a initial separation between streamers that is smaller or comparable to the longest characteristic absorption length of photoionization (0.1408 cm at the ground pressure), streamers will start to merge, i.e., the distance between their trajectories starts to decrease despite their electrostatic repulsion, at the moment when the ratio D/d of the streamer characteristic width D and the streamer mutual distance d attains a value of about 0.35 for positive streamers, and 0.4 for negative ones. These ratios are independent of the applied field, the initial seed configuration, and pressure. Moreover presented results clearly illustrate the significant role of photoionization on the streamer merging.