  Modes with a high-$Q$ factor are crucial for photonic metadevices with
advanced functionalities. In sharp contrast to recent techniques which generate
a supercavity mode by bound states in the continuum via symmetry breaking, we
reveal a general and new route, by stacking two parallel and identical
Mie-resonant metasurfaces with an air separation. The supercavity mode can be
designed by the established theoretical model by overlapping Mie resonant modes
with tailor-made Fabry-P\'{e}rot modes. The simplified system, with free-space
field concentration which can exist in plane as well as out of plane of the
metasurfaces, provides for ease of integration with added matter, creating
exciting different opportunities for the study of fundamental light-matter
coupling. This work deepens our understanding of light manipulation using
metasurfaces. It paves a different and general route for generating supercavity
modes which can be easily engineered and controlled for different applications
of all dielectric metaoptics.
