Catalysis of the reaction: S-adenosyl-L-methionine + phosphatidyl-N-methylethanolamine = S-adenosyl-L-homocysteine + phosphatidyl-N-dimethylethanolamine. Also catalyzes the transfer of a further methylgroup, producing phosphatidylcholine.
phosphatidyl-N-monomethylethanolamine methyltransferase activity