  In the Milky Way, recent progress in the exploration of its assembly history
is driven by the tremendous amount of high-quality data delivered by Gaia,
which has revealed a number of substructures potentially linked to several
ancient accretion events. In this work, aiming to explore the phase-space
structure of accreted stars, we analyze six M31/MW analogues from the HESTIA
suite of cosmological hydrodynamics zoom-in simulations of the Local Group. We
found that all the HESTIA galaxies experience a few dozen mergers but only 1-4
mergers have the stellar mass ratio >0.2 where, depending on the halo
definition, the most massive merger contributes from 20% to 70% of the total
stellar halo. Individual merger remnants show diverse density distributions at
z=0, significantly overlapping with each other and with the in-situ stars in
the ELz, UV and RVphi coordinates. The mergers debris often change their
position in the ELz with time due to the galactic mass growth and the
non-axisymmetry of the potential. In agreement with previous works, we show
that even individual merger debris exhibit a number of distinct ELz features.
In the UV plane, all HESTIA galaxies reveal radially hot, non-rotating or
weakly counter-rotating, Gaia-Sausage-like features. We found an age gradient
in Elz space for the individual debris, where the youngest stars, formed in the
inner regions of accreting systems, deposit to the innermost regions of the
host. The bulk of these stars is being formed during the last stages of
accretion, making it possible to date the merger. In actions space (Jr, Jz,
J\phi), the mergers debris do not appear as isolated substructures but are
instead scattered over a large parameters area and overlapping with the in-situ
stars. We also introduce a purely kinematic space (Jz/Jr-eccentricity), where
different merger debris can be disentangled better from each other and from the
in-situ stars.
