Arginine methyltransferase involved in the assembly or stability of mitochondrial NADH:ubiquinone oxidoreductase complex (complex I). L-arginyl-[protein] + 2 S-adenosyl-L-methionine = 2 H(+) + N(omega),N(omega)'-dimethyl-L-arginyl-[protein] + 2 S-adenosyl-L-homocysteine Belongs to the NDUFAF7 family.