Catalysis of the reaction: N(6)-L-threonylcarbamoyladenine(37) in tRNA + sulfur-(sulfur carrier) + 2 S-adenosyl-L-methionine = 2-methylthio-N(6)-L-threonylcarbamoyladenine(37) in tRNA + S-adenosyl-L-homocysteine + (sulfur carrier) + L-methionine + 5'-deoxyadenosine.
tRNA (N(6)-L-threonylcarbamoyladenosine(37)-C(2))-methylthiotransferase