You destroy your targeted card and return your opponent’s targeted cards to the hand at the same time.
If control of one or more of the targets is switched before this effect resolves, this effect does nothing.
If your opponent controls the card you targeted to be destroyed when this effect resolves, it is not destroyed and you don’t return any cards to your opponent’s hand.
When resolving the effect of "Scrap Twin Dragon", if the targeted monster on your side of the field cannot be destroyed because it is no longer on the field, then you cannot return your opponent's cards to the hand. Also, if either one of the opponent's targeted cards is no longer on the field, then the card on your field must still be destroyed.
The "destroy the one you control" and "return the two your opponent controls to the hand" effects are treated as resolving simultaneously.
If control of one or more of the targets is switched before this effect resolves, then you neither destroy your card nor return the opponent's cards to his hand.