Cubic A2B2O7 oxides with A standing for a rare-earth element and B for a transition or main block metal have been systematically studied for their frequently exotic crystallographic and electronic properties. The majority of A2B2O7 compounds crystallize in a cubic pyrochlore or defect-fluorite structure. In the proposed experiment we focus on Er2B2O7 oxides with B = Zr and Ir, adopting long-range defect-fluorite and pyrochlore structures, respectively. Despite structural differences, both the zirconate and iridate show surprisingly similar spin-glass-like magnetic transitions at low temperature. We aim to use neutron total scattering to probe the local structures of these materials and to help determine whether the similar magnetic responses are due to similar local structures or truly different frustration processes (fluorite structural disorder vs. pyrochlore geometric frustration).