To clarify, are there two different people in your party that are in need of the
DAS?
When obtaining 2 cards, your party will be split between the two cards. However, outside of the DAS holder, the remainder of your party is not "assigned" to a card. With the party of 9, you could have the DAS holder plus 3 additional guests on one card and the other DAS holder plus 4 additional guests on the other card. But there's nothing that says the non-disabled teens are only on the card with the DAS holder who is a teen. (I hope that makes sense.)
The general rule is that all members of the party must be there when the DAS is obtained if you wish to have a party of more than 6 on the DAS card. With 2 disabled individuals, as long as they each qualify for the DAS, you could approach it as two separate groups. One parent goes with the first disabled individual and gets a DAS for them with a party of 5 listed (DAS holder + 4 additional). Another parents takes the second disabled individual and requests a DAS for a party of 4 (DAS holder + 3 individuals). Since each of these groups is under 6 people, not everyone would have to be present that first day.
If there is only one person in your party who will qualify for the DAS, then you will only get one DAS card and everyone in your party will have to be present. On the first day, everyone goes to GS together and requests the DAS for a party of 7. Then on the second day, when the additional 2 people are present, return to GS with the DAS card reissued the day before and all 9 people in your party. The DAS can then be issued for a party of 9.