  Massive merging galaxy clusters often host diffuse Mpc-scale radio
synchrotron emission. This emission originates from relativistic electrons in
the ionized intracluster medium (ICM). An important question is how these
synchrotron emitting relativistic electrons are accelerated. Our aim is to
search for diffuse emission in the Frontier Fields clusters Abell S1063 and
Abell 370 and characterize its properties. While these clusters are very
massive and well studied at some other wavelengths, no diffuse emission has
been reported for these clusters so far. We obtained 325 MHz Giant Metrewave
Radio Telescope (GMRT) and 1--4 GHz Jansky Very Large Array (VLA) observations
of Abell S1063 and Abell 370. We complement these data with Chandra and
XMM-Newton X-ray observations. In our sensitive images, we discover radio halos
in both clusters. In Abell S1063, a giant radio halo is found with a size of
$\sim 1.2$ Mpc. The integrated spectral index between 325 MHz and 1.5 GHz is
$-0.94\pm0.08$ and it steepens to $-1.77 \pm 0.20$ between 1.5 and 3.0 GHz.
This spectral steepening provides support for the turbulent re-acceleration
model for radio halo formation. Abell 370 hosts a faint radio halo mostly
centred on the southern part of this binary merging cluster, with a size of
$\sim 500-700$ kpc. The spectral index between 325 MHz and 1.5 GHz is
$-1.10\pm0.09$. Both radio halos follow the known scaling relation between the
cluster mass proxy $Y_{500}$ and radio power, consistent with the idea that
they are related to ongoing cluster merger events.
