  We use MUSE adaptive optics (AO) data in Narrow Field Mode to study the
properties of the ionised gas in MR 2251-178 and PG 1126-041, two nearby
(z~0.06) bright quasars hosting sub-pc scale Ultra Fast Outflows (UFOs)
detected in the X-ray band. We decompose the optical emission from diffuse gas
into a low- and a high-velocity components. The former is characterised by a
clean, regular velocity field and a low (~80 km/s) velocity dispersion. It
traces regularly rotating gas in PG 1126-041, while in MR 2251-178 it is
possibly associated to tidal debris from a recent merger or flyby. The other
component is found to be extended up to a few kpc from the nuclei, and shows a
high (~800 km/s) velocity dispersion and a blue-shifted mean velocity, as
expected from AGN-driven outflows. We estimate mass outflow rates up to a few
Mo/yr and kinetic efficiencies between 0.1-0.4 per cent, in line with those of
galaxies hosting AGNs of similar luminosity. The momentum rates of these
ionised outflows are comparable to those measured for the UFOs at sub-pc
scales, consistent with a momentum-driven wind propagation. Pure energy-driven
winds are excluded unless about 100x additional momentum is locked in massive
molecular winds. By comparing the outflow properties of our sources with those
of a small sample of well-studied QSOs hosting UFOs from the literature, we
find that winds seem to systematically lie either in a momentum-driven or in an
energy-driven regime, indicating that these two theoretical models bracket very
well the physics of AGN-driven winds.
