With the increase in non-communicable diseases, cancer is becoming one of the most lethal ailments of the coming decades. Significant progress has been made in the development of NPs that combine diagnostic and therapeutic properties in a single system. Multimodal NPs that sequentially perform MRI diagnostics with increased contrast and then act as synergistic agents for magnetic hyperthermia and radiotherapy can be considered as next-generation anticancer drugs. Thus, we propose a systematic study of composite theranostic ZnFe2O4@MnFe2O4 NPs for the first time. Two types of magnetic NPs with MnFe2O4 shell thicknesses of 0.5 (ZM0.5) and 1.7 nm (ZM3) were prepared via hydrothermal synthesis. Tuning the shell thickness was shown to influence the NP r2 and r1 relaxivities and allow T1-T2 dual-mode contrast agents to be obtained. A radiotherapy study demonstrated a significant dose factor enhancement (about 40%) for both NP types. The specific absorption rate of ZM3 in a 100 Oe alternating magnetic field with a frequency of 75 kHz was found to be 8 W g-1, which results in heating up to 42 °C within a few seconds. This work presents high-performance multifunctional NPs capable of combining different diagnostic and therapeutic methods for a full course of treatment using only one type of NP.