Top-left panel: Ionized gas kinematics of UG101 and UG101b. Only spaxels with (S/N)_Hα ≥ 4 are shown, and the systemic velocity of each galaxy is subtracted. In gray, we represent the slit element used to derive rotation curves (see text), which is aligned to the kinematic semimajor axis. Top-right panel: Gas-phase velocity dispersion of both galaxies, traced by their rest-frame Hα emission. Displayed values are corrected for instrumental broadening. Bottom-left panel: Stellar velocity field of both galaxies, derived with PPXF (see text). Bottom-right panel: Stellar velocity dispersion. For better visualization, the stellar velocity and velocity dispersion maps are smoothed using a local weighted regression, as implemented by the LOESS2D Python package (Cappellari et al. 2013).