Hi Watford Girl - we're in a similar position for the boys grammar, and yes I believe they go in criterion order, so when they get to the selective places, 1st they look for their top music score applicants, then their academic list.
Some DCs will be high on both, but would be offered a music place in that case, allowing another academic entrant to take the other slot.
It says on both grammar websites, "Applicants who are entitled to be offered a place under more than one of criteria 3, 4, 5, 6, 7 or 8, will be offered a place under the lowest numbered criterion".
Sorry I've just realised you might not be talking about the girls grammar! But probably other SW Herts schools are similar!
That's very helpful thank you!