We present the experimental features of second harmonic emission from an air plasma coexcited by 2 ultrafast laser pulses and propose a theoretical model that describes the observed properties. The dual-beam optical nonlinear process was discovered to depend on the optical polarization of both fundamental pulses simultaneously

and the output emissions coincided with the propagation directions of the 2 input laser beams. These attributes of the ultrafast phenomenon are reconciled by incorporating the action of plasma grating into the interaction picture. When the diffractive property of the laser-induced grating is considered

each of the fundamental beams is diffracted onto the direction of the other. The collinear fundamental components can then generate a second harmonic radiation that fulfills the phase matching condition. The ensuing theoretical simulation fits well against the experimental observations. Finally

we analyze the effective nonlinear susceptibility tensor of the air plasma to facilitate future research on the hidden symmetries within plasmas induced by intense ultrafast lasers.