In the two-step synthesis, first the binary selenides were prepared. MgSe and Sc2Se3 were synthesized from stoichiometric amounts of the elemental powders according to the previous one-step synthesis at 750 °C for 24 h (180 °C h -1 heating rate) and 800 °C for 30 h (60 °C h -1 heating rate), respectively. Instead, the Tm2Se3, Er2Se3 and Y2Se3 syntheses were carried out with stoichiometric amounts of selenium powder and thulium, erbium or yttrium metal chips, respectively, which are less prone against oxidation than powders. The selenium powder was placed on the bottom of a graphite crucible and then covered with the metal chips. After sealing the crucible in an evacuated quartz glass ampule, heating at 800 °C for 48 h (60 °C h -1 heating rate) was performed. In the second synthesis step, MgSe was mixed and pelletized in a 1:1 ratio with either Sc2Se3, Tm2Se3, Er2Se3 or Y2Se3 to obtain the corresponding MgB2Se4 spinel by thermal conversion. For the MgSc2Se4 synthesis the same conditions as described in the one-step synthesis were applied, while a longer heating time of 30 h was used for the other spinels. All preparations and sample treatments were performed under argon atmosphere or in vacuum. version v2022.10.28: Tm2O2Se, (mp-753920) and Y2O2Se (mp-752658).