Flexible metal-organic frameworks (FMOFs) with "S-shaped" stepped isotherms are stimuli-responsive smart materials that exhibit dynamic structural changes in response to guests, offering intrinsic thermal management and excellent regenerability compared to the classical rigid adsorbents. Herein, a 2-fold interpenetrated 3D flexible MOF (FMOF): IITKGP-39 is reported whose gate-opening behavior is synchronously intertwined with the pore functionalization strategy, which served as a fascinating tool for simultaneous redressal of highly demanding one-step ethylene (C2H4) purification via inverse ethane-ethylene (C2H6/C2H4) separation (i.e., C2H6 trapping) as well as acetylene-carbon dioxide (C2H2/CO2) separation through one-step sorbent separation technology (OSST). Notably, IITKGP-39 represents the first FMOF to exhibit inverse C2H6/C2H4 sorption behavior with outstanding breakthrough separation selectivity of ∼2.3 at room temperature (298 K), accompanied by one-step C2H4 purification from ternary C2 gas mixtures. Further, it excels in C2H2/CO2 separation with a remarkable uptake ratio of ∼18 and high breakthrough selectivity of 8.3. A detailed elucidation into structural dynamics is revealed through in situ X-ray diffraction analysis that manifested the sorbate-induced structural transformation among three distinct phases: as-synthesized (phase-α), activated (phase-β), and gas sorption/gate opened phase (phase-γ). Thus, IITKGP-39 is a unique industrially compatible two-in-one physisorbent that serves as a multipurpose molecular separator by virtue of reversible flexibility with outstanding separation performances.