Engineering emergent ferromagnetism at correlated-oxide interfaces offers a powerful route to creating collective states that do not exist in the parent materials. Here, we show that interfacial valence reconstruction in NdNiO₃/CaMnO₃ superlattices generates dual-sublattice ferromagnetism involving both Mn and Ni. Depth-resolved standing-wave X-ray photoelectron spectroscopy reveals enhanced Mn³⁺ character on the CaMnO₃ side of the interface and enhanced Ni²⁺ character on the NdNiO₃ side, establishing the configurations required for Mn⁴⁺-O-Mn³⁺ double exchange and Ni²⁺-O-Mn⁴⁺ superexchange, respectively. At low temperature, element-specific XMCD reveals ferromagnetic responses from both sublattices, with the Ni response concentrated predominantly in the Ni²⁺-derived spectral component that SW-XPS independently shows to be enhanced at the interface. Together, these results show that cooperative double exchange within CaMnO₃ and superexchange across the interface couple the Mn and Ni sublattices, providing a general strategy for engineering interfacial ferromagnetism in correlated oxides.