Heterogeneous integration combines complementary material properties in a single photonic platform. Here, we demonstrate on-chip f-2f self-referencing in a heterogeneously integrated Si₃N₄/LiNbO₃ platform, in which supercontinuum generation in a Si₃N₄ waveguide and second-harmonic generation in a LiNbO₃ layer are spatially separated and linked by adiabatic escalator couplers. Pumped by a 1560 nm mode-locked laser with 75 pJ on-chip pulse energy, we detect the carrier-envelope offset frequency with 30 dB signal-to-noise ratio in 300 kHz resolution bandwidth. A comparative measurement confirms that the second harmonic originates in the LiNbO₃ layer and indicates opportunity for further improvements through periodic poling. These results establish heterogeneous Si₃N₄/LiNbO₃ integration as a viable building block for self-referenced ultrafast pulse sources in low-loss Si₃N₄ photonics.