Metal nanoparticles supported on carbon nanofibers (CNFs) are promising for catalytic and electrochemical applications, but controlling their size, dispersion, and phase during rapid thermal processing remains challenging. Here, we report a dual-solvent microwave carbothermal shock (CTS) method for producing Cu- and Ni-containing nanostructures on CNFs under ambient-pressure conditions. Ethanolic metal nitrate precursors were deposited on pristine CNFs, and hexane was introduced as an antisolvent before microwave irradiation. At 0.05 M Cu(NO₃)₂, hexane addition decreased the mean Cu-containing particle diameter from 50 ± 20 nm to 33 ± 12 nm and improved dispersion. Varying precursor concentration further altered particle morphology and aggregation. For Ni, single-solvent processing yielded aggregated nanoparticles averaging 43 ± 16 nm, whereas dual-solvent processing produced a dense carbon nanotube network with an apparent mean filament diameter of 20 ± 7 nm. Microscopy and elemental mapping supported the association of Ni-containing particles with the nanotubes. X-ray diffraction identified crystalline face-centered cubic Cu and Ni, while X-ray photoelectron spectroscopy indicated oxidized surface species. Hexane may additionally supply carbon during Ni-assisted nanotube growth, although its contribution requires confirmation. This approach provides a rapid, tunable route to supported metal nanoparticles and hierarchical carbon nanostructures without continuous external inert or hydrocarbon gas flow.