Well thats assuming if it would have dispersed that far. Only naruto and sasuke managed to pass through juubi babies.I dont think it wouldve made sesne to do it like that, because if the amaterasu was first then naruto FRS couldve dispersed the amaterasu among the alliance or anyone already in side the barrier with them.
Ur forgetting that fire consumes wind.I think the way they did it in the manga was best. Remember the way FRS works, it sends thousands of bolts of tiny chakra blades, sharp as wind all throughout the opponents body. Now imagine that, except those tiny sharp chakra bolts are also infused with the powerful flames of the amaterasu. every cell of the juubi is burning right now in amaterasu.