GO:0035598:   N6-threonylcarbomyladenosine methylthiotransferase activity
Catalysis of the methylthiolation (-SCH3 addition) at the C2 of the adenosine ring of N6-threonylcarbomyladenosine (t6A) in tRNA, to form 2-methylthio-N6-threonylcarbamoyladenosine (ms2t6A).