Catalysis of the reaction: 4 S-adenosyl-L-methionine + adenine(1518)/adenine(1519) in 16S rRNA = 4 S-adenosyl-L-homocysteine + N(6)-dimethyladenine(1518)/N(6)-dimethyladenine(1519) in 16S rRNA.
S-adenosylmethionine-6-N\',N\'-adenosyl (rRNA) dimethyltransferase activity