We consider two parallel dielectric gratings (dual grating) which support accidental bound states in the continuum (BICs) mostly localized between gratings. As distinctive to true periodical BICs in an infinite dual grating, the enveloping intensity of quasi-BICs in a finite dual grating behaves as a standing wave. That behavior is a key property to trap nanoparticles into selected cells of the dual grating sorted by sizes of nanoparticles dragged by liquid flowing between gratings. For excitation of quasi-BIC with high quality factor by an electromagnetic plane wave with normal incidence and power 1 mW/µm2 we show high efficiency of sorting of nanoparticles by sizes.