  Two years after InSight's arrival at Mars, the ExoMars 2020 mission will land
on the opposite side of the red planet. Similarly to InSight, which carries the
RISE (Rotation and Interior Structure Experiment) radio-science experiment, the
ExoMars mission will have on board the Lander Radio-science (LaRa) experiment.
The X-band transponders on RISE and LaRa, allowing for direct radio-link
between the landers and stations on Earth, are dedicated to the investigation
of Mars' deep interior through the precise measurement of the planet's rotation
and orientation. The benefit of having LaRa after RISE for the determination of
the Mars orientation and rotation parameters is demonstrated and the resulting
improved constraints on the interior structure of Mars and, in particular, on
its core are quantified via numerical simulations. In particular, we show that
the amplitudes of the semi-annual prograde ($p_2)$ and the ter-annual
retrograde ($r_3$) nutations will be determined with a precision of 6 and 4
milliarcseconds respectively by combining 700 days of RISE data with 700 days
of LaRa data, about 35$\%$ more precise than what is expected from RISE alone.
The impact of such an improvement on the determination of the core size of Mars
is discussed and shown to be significant.
