  Using $HST$/COS observations of the twin quasar lines of sight Q$0107-025$A
$\&$ Q$0107-025$B, we report on the physical properties, chemical abundances
and transverse sizes of gas in a multiple galaxy environment at $z = 0.399$
across a transverse separation of $520$ kpc. The absorber towards Q$0107-025$B
has $\log N(H I)/cm^{-2} \approx 16.8$ (partial Lyman limit) while the absorber
towards the other sightline has $N(H I) \approx 2$ dex lower. The O VI along
both sightlines have comparable column densities and broad $b$-values, whereas
the low ionization lines are considerably narrower. The low ionization gas is
inconsistent with the O VI when modelled assuming photoionization in a single
phase. Along both the lines-of-sight, O VI and coinciding broad H I are best
explained through collisional ionization in a cooling plasma with solar
metallicity. Ionization models infer $1/10$-th solar metallicity for the pLLS
and solar metallicity for the lower column density absorber along the other
sightline. Within $\pm~250~km~s^{-1}$ and $2$ Mpc of projected distance from
the sightlines 12 galaxies are identified, of which 3 are within $300$ kpc. One
of them is a dwarf galaxy while the other two are intermediate mass systems at
impact parameters of $\rho \sim (1-4)R_{vir}$. The O VI along both
lines-of-sight could be either tracing narrow transition temperature zones at
the interface of low ionization gas and the hot halo of nearest galaxy, or a
more spread-out warm gas bound to the circumgalactic halo/intragroup medium.
This latter scenario leads to a warm gas mass limit of $M \gtrsim 4.5 \times
10^{9}$ M$_\odot$.
