  Huygens' metasurfaces have demonstrated a remarkable potential to perform
wave transformations within a subwavelength region. In particular,
omega-bianisotropic Huygens' metasurfaces have allowed for the passive
implementation of any wave transformation that conserves real power locally.
Previous reports have also shown that Huygens' metasurface pairs are capable of
realizing transformations that break the local power conservation requirement
by redistributing the total power, while the wave propagates between the two
metasurfaces. However, the required separation distance overshadows the
low-profile characteristics of the individual metasurfaces and leads to bulky
designs, especially for lower frequencies. In this work, we develop a method of
designing omega-bianisotropic Huygens' metasurface pairs, relying on a
point-matching process of the real power at the two metasurfaces. We highlight
the versatility of our method by presenting two variations of the
configuration, depending on whether the electromagnetic source is located
within or outside the metasurface pair. Based on the examples of a
cylindrical-wave to plane-wave transformation and a beam expander, we examine
the impact of multiple reflections, as a way to overcome the size limitations
and design compact structures. Moreover, we explore possible beamforming
applications through an example of a Taylor-pattern antenna with a single
feed-point between the two metasurfaces.
