I don't know why you want cheat on this game? This is probably the simplest and easiest game and you still want to cheat? please

Sometimes it has to be different dwellers... I just test this by checking each room has about the same level to begin with (low) and rename 1 of them to keep track of how many times it has upgraded already. If the second time goes alright, then I'll let them train further, if not swap around..

Not sure what the difference is tho between some objectives like this, i.e. when it's going to be unique dwellers one time or the same ones twice