Catalyzes the methylation of both C-5 and C-15 in precorrin-6Y to form precorrin-8X. precorrin-6B + 2 S-adenosyl-L-methionine = CO2 + 3 H(+) + precorrin-8X + 2 S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from precorrin-2 (aerobic route): step 7/10. Belongs to the precorrin methyltransferase family.