Forms the bisbenzylisoquinoline alkaloid berbamunine by phenol oxidation of N-methylcoclaurine without the incorporation of oxygen into the product. Oxidatively couples either two molecules of (R)-N-methylcoclaurine to form the (R,R) dimer guattegaumerine or one molecule each of (R)- and (S)-N-methylcoclaurine to form the (R,S) dimer berbamunine. (R)-N-methylcoclaurine + (S)-N-methylcoclaurine + O2 + reduced [NADPH--hemoprotein reductase] = berbamunine + H(+) + 2 H2O + oxidized [NADPH--hemoprotein reductase] Alkaloid biosynthesis; berbamunine biosynthesis; berbamunine from (R)-N-methylcoclaurine and (S)-N-methylcoclaurine: step 1/1. Belongs to the cytochrome P450 family.