O-methyltransferase that catalyzes the 2 O-methylation steps in the ubiquinone biosynthetic pathway. a 3-demethylubiquinol + S-adenosyl-L-methionine = a ubiquinol + H(+) + S-adenosyl-L-homocysteine a 3-(all-trans-polyprenyl)benzene-1,2-diol + S-adenosyl-L-methionine = a 2-methoxy-6-(all-trans-polyprenyl)phenol + H(+) + S-adenosyl-L-homocysteine Cofactor biosynthesis; ubiquinone biosynthesis. Homodimer (Probable). Component of the Ubi complex metabolon, which regroups five ubiquinone biosynthesis proteins (UbiE, UbiF, UbiG, UbiH and UbiI) and two accessory factors (UbiK and the lipid-binding protein UbiJ) (PubMed:30686758). Belongs to the methyltransferase superfamily. UbiG/COQ3 family.