The Longitudinal Valley Fault (LVF) and the Central Range Fault (CRF) in eastern Taiwan are hosts of disastrous earthquakes. These are closely located and dip in opposite directions (inverted V-shaped). It was previously proposed that earthquakes on one of these faults suppress earthquakes on the other. Nonetheless, the 2022 Chihshang earthquake (M_w 7.0) on the CRF occurred soon after the 2022 Yuli earthquake (M_w 6.7) on the LVF. We comprehensively understand these contradictory findings by focusing on the fault slip directions. First, we revealed almost pure reverse faulting of the LVF during the Yuli earthquake from geodetic observations. According to our simulation, the Yuli earthquake imparted the stress equivalent to a steady stress accumulation of 36–100 years around the main rupture area of the Chihshang earthquake on the CRF, suggesting a large impact on the earthquake generation. Finally, we demonstrated the importance of slip directions on the fault interaction within the inverted V-shaped fault systems: reverse slip prompts earthquakes on the other fault while lateral slip suppresses. The effect of the slip direction consistently explains both the previously suggested seismic quiescence and the promotion of the Chihshang earthquake by the Yuli earthquake. This mechanism is also consistent with historical seismicity.