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The striking contrast between deeply incised large rivers and low-relief interfluve surfaces in Southeast Tibet presents a compelling geomorphic enigma. The onset of rapid river incision in this region, inferred from low-temperature thermochronometric data, has often been interpreted as indicative of the timing of regional surface uplift; however, these findings contradict those derived from paleo-altimetric studies. The origins of the low-relief, high-elevation interfluve surfaces have likewise been the subject of prolonged debate. These controversies underscore the difficulty of unraveling the intricate relationships between drainage evolution, climate, local tectonics, and regional surface uplift in Southeast Tibet. This challenge is further complicated by the fact that most available low-temperature thermochronometric datasets are sourced from isolated transects separated by tens to hundreds of kilometers or from roadside sampling within this rugged and often inaccessible terrain. In this study, we present low-temperature thermochronometric data collected from two adjacent tributary catchments of the upper Mekong River, which provide fresh insights into the timing and mechanisms underlying river incision in this area. Thermal history modeling indicates that accelerated denudation driven by river incision began at the Oligocene-Miocene transition (22-26 Ma). This timing postdates the principal phase of Eocene regional surface uplift but coincides with an intensification of the Southeast Asian monsoon. While one tributary catchment shows relatively stable denudation rates of approximately 250 m/Myr throughout the Neogene, the neighboring catchment experienced a higher rate of about 350 m/Myr from 26 to 8 Ma, followed by a order-of-magnitude decline in denudation rate after approximately 8 Ma. This heterogeneous denudation history reflects dynamic post-orogenic drainage integration, with the decrease in denudation rate attributed to upstream tributary capture. Our findings elucidate a two-stage evolution model of drainage and topographic relief, highlighting the decoupling between surface uplift and exhumation. This model presents an alternative perspective to previous conflicting interpretations regarding the formation and dissection of low-relief surfaces in Southeast Tibet.