Dimethylates a single guanine residue at position 26 of a number of tRNAs using S-adenosyl-L-methionine as donor of the methyl groups. guanosine(26) in tRNA + 2 S-adenosyl-L-methionine = 2 H(+) + N(2)-dimethylguanosine(26) in tRNA + 2 S-adenosyl-L-homocysteine Monomer. Belongs to the class I-like SAM-binding methyltransferase superfamily. Trm1 family.