We employed density functional theory, Monte Carlo simulations and a mean field model to study phase separation in thermoelectric Ni(Ti,Zr)(Sb,Sn) half-Heusler materials, simultaneously alloyed in the (Ti,Zr)- and (Sb,Sn) sublattices. We found that the material shows demixing and ordering phenomena as the temperature is lowered. Below a critical temperature, which depends on the overall stoichiometry, demixing occurs within the sublattices. Typically, a strong demixing in the (Ti,Zr) sublattice is accompanied by a weaker demixing in the (Sb,Sn) sublattice stoichiometry. In the coexistence region, the Sb concentration in the Ti-rich phase is higher than that in the Zr-rich phase. For a Sn : Sb ratio of 1 : 3, we find a second phase separation in the Sb/Sn sublattice at about 200 K. Here, a phase without Sn coexists with a phase that has a 1 : 1 ratio of Sb and Sn. In the latter phase Sb and Sn alternate within the sublattice forming a highly ordered structure. The results provide new insights into the pattern formation in Ni(Ti,Zr)(Sb,Sn) which helps in creating nano-structuring strategies to improve the figure of merit in this class of thermoelectric materials.