We present concerted local and overall crystal structure studies on Ho_1-xDy_xMnO₃ specimens (x = 0 to 1), using synchrotron powder XRD and FTIR, to probe the underlying microscopic changes during hexagonal to orthorhombic phase evolution. In orthorhombic phases, cationic substitution has stronger impact on MnO₆ octahedraltilt and Mn–O–Mn bond-angles keeping Mn–O subsystem almost unaltered unlike the cases of hexagonal phases, where MnO₅ bipyramid suffers significant distortion due to their interconnected (with R³⁺ ion) layered structure. Ionic radii mismatch induces disorder (σ²) on A-site and results in a significant amount of mixed phase region in terms of <r_A> and t (1.0198 ≤ <r_A> ≤ 1.0246 Å; 0.8354 ≤ t ≤ 0.8371) at the cost of pure phases. A comparison between the phases of Ho_1-xDy_xMnO₃ and other similar compounds like (Y,R)_hR_oMnO₃ (YMnO₃, R_hMnO₃ - hexagonal and R_oMnO₃ - orthorhombic) compounds reveals that mixed phase region expands linearly with σmax in such doped manganites.