In this work, measurement in the near-field region of a sub-wavelength separation between two atoms is studied when the detector used to perform the measurement is modeled as another atom. Through an analysis of multiple scattering of an incident wave among the atoms, the back-action of the detector on the measurement is examined. It is found that, although the separation can never be measured exactly, the presence of the back-action makes the measurement more accurate. It is also found that the back-action enables the waves observed by the detector to avoid singularities that otherwise exist when the back-action is ignored.