Supplies decaprenyl diphosphate, the precursor for the side chain of the isoprenoid quinones ubiquinone-10. (2E,6E)-farnesyl diphosphate + 7 isopentenyl diphosphate = all-trans-decaprenyl diphosphate + 7 diphosphate Binds 2 Mg(2+) ions per subunit. Cofactor biosynthesis; ubiquinone biosynthesis. Heterotetramer of 2 dps1 and 2 dlp1 subunits. Belongs to the FPP/GGPP synthase family.