The effect of isovalent substitution of rare-earth cations on the phase separation in half-doped manganites was studied on the example of Sm0.32Pr0.18Sr0.5MnO3 by high-resolution neutron powder diffraction, neutron beam depolarization, second-harmonic magnetic response, and magnetization and resistivity measurements from 4 K up to room temperature and higher. A structural phase transition from the orthorhombic Pbnm phase to a mixture of Pbnm and monoclinic P21/m phases was observed upon cooling. The magnetic ground state was found to be phase-separated into three magnetic phases emerging at different temperatures, viz., ferromagnetic (FM) and antiferromagnetic A and charge-ordered CE types. FM clusters arise far above room temperature in the orthorhombic phase and coalesce upon cooling to produce the long-range FM order below 250 K and to arrive at the percolative FM phase below 120 K. The A- and CE-type orders form in the monoclinic phase at the temperatures 200 K and 120 K, respectively. The Sm/Pr isovalent substitution qualitatively changes the phase separation and significantly increases its temperature range compared to the parent compounds. The results obtained give us knowledge of phase separation occurring in systems with strong electron correlations and extend opportunities for fine-tuning of their properties.