  We investigate magnetism and band structure engineering in Moir\'e
superlattice of blue phosphorus ($\beta$-P) and grey arsenene ($\beta$-As)
bilayers, using \textit{ab initio} calculations. The electronic states near the
valence and conduction band edges have significant $p_z$ character in both the
bilayers. Thus, twisting the layers significantly reduce the interlayer orbital
overlap, leading to a decrease in the binding energy (up to $\sim33\%$) and an
increase in interlayer distance (up to $\sim10\%$), compared to the most stable
AA-stacking. This interlayer decoupling also results in a notable increase (up
to $\sim$25-50\%) of the bandgap of twisted bilayers, with the valance band
edge becoming relatively flat with van-Hove singularities in the density of
states. Thus, hole doping induces a Stoner instability, leading to
ferromagnetic ground state, which is more robust in Moir\'e superlattices, than
that of AA-stacked $\beta$-P and $\beta$-As.
