Catalysis of the reaction: 4 S-adenosyl-L-methionine + adenine(1779)/adenine(1780) in 18S rRNA = 4 S-adenosyl-L-homocysteine + N(6)-dimethyladenine(1779)/N(6)-dimethyladenine(1780) in 18S rRNA.
M(6)(2)A dimethylase activity