Catalysis of the reaction: guanosine(26)/guanosine(27) in tRNA + 4 S-adenosyl-L-methionine = 4 H+ + N(2)-dimethylguanosine(26)/N(2)-dimethylguanosine(27) in tRNA + 4 S-adenosyl-L-homocysteine.
tRNA (guanosine(26)-N(2)/guanosine(27)-N(2))-dimethyltransferase activity