The existence of a filling fraction limit (FFL) for single-element filling in caged compounds such as CoSb3 has been established as a general phenomenon by both experiment and theory. For dual-element-filled skutterudites, our ab initio results reveal that the filling fraction (FF) of one filler usually decreases linearly as the other increases, and the total FF (the sum of the FFs of the two fillers) lies between the two single-element FFLs. However, the Na-Eu combination manifests an anomalous behavior: the total FF can exceed both the Na-only and Eu-only FFLs and is even likely to reach full filling. Electronic structure analysis shows that the Na-Eu cofilling pushes the s states of Na and Eu closer to the bottom of the conduction band, increases the electron occupancy of these s orbitals, lowers the effective charge states of the fillers, affects the Na-Eu interaction, and finally leads to the anomalous filling behavior.