A dual-beam intersection apparatus for microwave breakdown plasma spectroscopy (MBS) has been developed and tested for real-time, simultaneous, and in situ monitoring of the trace gases in the air. The dual microwave beams, which are supplied by a 9.4G Hz pulse magnetron (feed power: 0--80 kW, pulse width: 5 ¿s, pulse repetition frequency: 10 Hz) using a power divider, are focused and superposed on a center of a reaction chamber through two dielectric lenses. The superposed electric field brings about the gaseous breakdown phenomena on the focusing point, where the photons emitted from the proper gas components can be detected and assigned using two-dimensional wavelength/time-resolved photoemission spectroscopy. The preliminary experiment has demonstrated that the emission intensities of breakdown plasma strongly depended on the phase difference between two microwave beams. Also, almost all the plasma spectra of the dual-beam system were more intensified than that of the single-beam system developed previously by us, and the relative continuum background decreased remarkably as well. For the 81 ppm of CCl2F2 gas mixed with the model atmosphere, the Cl atomic line of 837.6 nm (4p4D7/20 - 4s4P5/2) could clearly be detected at room temperature in total pressure of 10Torr. Present experiments have proved that the beam controllability of the dual-beam system is superior to that of the single-beam system to realize the appropriate breakdown conditions.