  Disorder free many-body localization (MBL) can occur in interacting systems
that can dynamically generate their own disorder. We address the thermal-MBL
phase transition of two isotropic Heisenberg spin chains that are
quasi-periodically coupled to each other. The spin chains are incommensurate
and are coupled through a short range exchange interaction of the $XXZ$ type
that decays exponentially with the distance. Using exact diagonalization,
matrix product states and density matrix renormalization group, we calculate
the time evolution of the entanglement entropy at long times and extract the
inverse participation ratio in the thermodynamic limit. We show that this
system has a robust MBL phase. We establish the phase diagram with the onset of
MBL as a function of the interchain exchange coupling and of the
incommensuration between the spin chains. The Ising limit of the interchain
interaction optimizes the stability of the MBL phase over a broad range of
incommensurations above a given critical exchange coupling. Incorporation of
interchain spin flips significantly enhances entanglement between the spin
chains and produces delocalization, favoring a pre-thermal phase whose
entanglement entropy grows logarithmically with time.
