  The simulation of the interaction of two simultaneously propagating air
streamers of the same polarity is presented. A parametric study has been
carried out using an accurate numerical method which ensures a time-space error
control of the solution. For initial separation of both streamers smaller or
comparable to the longest characteristic absorption length of photoionization
in air, we have found that the streamers tend to merge at the moment when the
ratio between their characteristic width and their mutual distance reaches a
value of about 0.35 for positive streamers, and 0.4 for negative ones. Moreover
it is demonstrated that these ratios are practically independent of the applied
electric field, the initial seed configuration, and the pressure.
