Bisectors determining unique pairs of points in the bidisk

Bisectors are equidistant hypersurfaces between two points and are basic objects in a metric geometry. They play an important part in understanding the action of subgroups of isometries on a metric space. In many metric geometries (spherical, Euclidean, hyperbolic, complex hyperbolic, to name a few) bisectors do not uniquely determine a pair of points, in the following sense\,: completely different sets of points share a common bisector. The above examples of this non-uniqueness are all rank $1$ symmetric spaces. However, as we show in this paper, bisectors in the usual $L^2$ metric are such for a unique pair of points in the rank $2$ geometry $\mathbb{H}^2 \times\mathbb{H}^2$.