Involved in the biosynthesis of forosamine ((4-dimethylamino)-2,3,4,6-tetradeoxy-alpha-D-threo-hexopyranose), a highly deoxygenated sugar component of several bioactive natural products such as the insecticidal spinosyns A and D (PubMed:11358695, PubMed:18345667). Catalyzes the dimethylation of the C-4 amino group from dTDP-4-amino-2,3,4,6-tetradeoxy-alpha-D-glucose to yield dTDP-D-forosamine (PubMed:18345667). dTDP-4-amino-2,3,4,6-tetradeoxy-alpha-D-erythro-hexopyranose + 2 S-adenosyl-L-methionine = dTDP-alpha-D-forosamine + 2 H(+) + 2 S-adenosyl-L-homocysteine Homodimer. Belongs to the methyltransferase TylM1/DesVI family.