  We investigated the origin and gas properties of a simulated galaxy cluster
pair, to connect simulation predictions to the SRG/eROSITA X-ray observations
of the Abell 3391/95 field. The simulated system has been extracted from the
(352 cMpc/h)^3 volume of the Magneticum Pathfinder cosmological simulations at
z=0.07. We tracked back in time the main progenitors of the pair clusters and
surrounding groups to study the assembly history of the system and its
evolution. Similarly to the observed A3391/95 system, the simulated pair is
embedded in a complex network of gas filaments, with structures aligned over
more than 20 projected Mpc and the whole region collapsing towards the central
overdense node. The spheres of influence (3*R200) of the two main clusters
already overlap at z=0.07, but their virial boundaries are still physically
separated. The diffuse gas located in the interconnecting bridge closely
reflects the WHIM, with typical temperature of ~1 keV and overdensity $\sim
100$, with respect to the mean baryon density of the Universe, and lower
enrichment level compared to the ICM in clusters. We find that most of the
bridge gas collapsed from directions roughly orthogonal to the intra-cluster
gas accretion directions, and its origin is mostly unrelated to the two cluster
progenitors. We find clear signatures in the surrounding groups of infall
motion towards the pair, such as significant radial velocities and slowdown of
gas compared to dark matter. These findings further support the picture of the
Northern Clump (MCXC J0621.7-5242) cluster infalling along a cosmic gas
filament towards Abell 3391, possibly merging with it. We conclude that, in
such a configuration, the pair clusters of the A3391/95-like system are in a
pre-merger phase, and did not interact yet. The diffuse gas in the
interconnecting bridge is mostly warm filament gas, rather than
tidally-stripped cluster gas.
