Interfacial effects or/and magnetic/dielectric synergy can be utilized effective strategies to improve electromagnetic wave (EMW) attenuation. Herein, using CoNi nanospheres as precursor, core@shell CoNi@C nanospheres could be efficiently fabricated via a facile polymerization and subsequent thermal treatment. And core@shell CoNi/NiSe2/Co9Se8@C@MoSe2 and CoNi2S4/Co9S8@C@MoS2 flower-like multicomponent nanocomposites (MCNCs) were fabricated via hydrothermal reaction amongst the obtained CoNi@C, Mo and Se/S sources. The generated carbon acting as protective layer was verified to greatly restrain the selenization of CoNi nanospheres during the formation of MoSe2 nanosheets, generating CoNi/NiSe2/Co9Se8@C@MoSe2. And the complete sulfidation of CoNi alloys led to the formation of CoNi2S4/Co9S8@C@MoS2. Because of their boosted impedance matching, conductive and polarization loss abilities, the acquired flower-like MoX2 (X = Se and S)-based MCNCs displayed greatly improved EMW absorption characteristics (EMWACs) compared with their precursors. Especially, the extraordinary synergistic effect generated into the magnetic/dielectric MCNCs contributed to outstanding EMWACs of CoNi/NiSe2/Co9Se8@C@MoSe2 flower-like MCNCs. Accordingly, our findings provided a combined strategy to simultaneously make the most of magnetic/dielectric synergistic and interfacial effects for boosted conduction loss, polarization loss and EMWACs.