The wording on Unstoppable keyword in the CRB allows a DarT to activate from the order pool twice when it has a faceup order. The faceup order would be lost, being unusable by the Unstoppable keyword restriction, but it does seem legal. Your other heavy misses an order but there are justifiable, albeit rare, situations when doing this is beneficial.
By the wording on Unstoppable I think this is legal, but this is unlikely the intent.