Context : 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 .

Aims : 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 .

Methods : 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 .

Results : 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 \pm 0.08 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 \pm 0.09 .
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 .

Conclusions :