Catalyzes the methylthiolation of N6-(dimethylallyl)adenosine (i(6)A), leading to the formation of 2-methylthio-N6-(dimethylallyl)adenosine (ms(2)i(6)A) at position 37 in tRNAs that read codons beginning with uridine. [sulfur carrier]-SH + AH2 + N(6)-dimethylallyladenosine(37) in tRNA + 2 S-adenosyl-L-methionine = 2-methylsulfanyl-N(6)-dimethylallyladenosine(37) in tRNA + 5'-deoxyadenosine + [sulfur carrier]-H + A + 2 H(+) + L-methionine + S-adenosyl-L-homocysteine Binds 2 [4Fe-4S] clusters. One cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Monomer. Belongs to the methylthiotransferase family. MiaB subfamily.