  Motivated by an intention to apply partial wave analysis to systems
containing photons, we construct eigenvectors of the distance of separation
between two photons. A choice is made that makes the case of two photons most
closely resemble the case of two massive particles. The treatment is
relativistic. An Hermitian separation observable can then be defined, unlike a
position operator for a single photon, which cannot be defined. We test these
results on a model system of two photons meant to describe the initial or final
state of a scattering experiment. We find that the separation probability
density gives a meaningful picture of localization.
