Shannon,
You're going to have a great year with Bigger!
I am thoroughly enjoying it this year with my son.
You will do the program "as is" in the daily plans with both of your girls. The extension pack readings are meant to be read independently by the student that they're for. These are scheduled in the appendix.
To use the storytime box in the guide, you'll need to choose one book from each of the 9 genres of literature. HOD has packs that you could purchase - these are the "deluxe package" on the Bigger packages page. They are labeled Classic, Boy Interest, and Girl Interest. If you're not wanting to purchase these, you could find them at your library possibly. There are 4 titles listed for each genre in the appendix of Bigger (the one from each of the 3 packs plus another). We LOVE the storytime box in the plans. It has some fun ideas for interacting with the great book choices. You are reading aloud to your kids here.
For DITHOR, your kids will be doing the reading. You'll also be using books from the 9 genres here. This is where the level 2, 3, 4/5 packs come in. You could choose to use one of these packs, or you could get your own books as you go here. I'd highly reccommend getting Carrie's Sample Book List, too. It has great ideas of good books to use for DITHOR - which will work with any book you choose. I don't think that the Extension Pack books would work well here because the different genres aren't represented. You can choose how often you do DITHOR, too. If you do it daily, you'll cover all 9 genres in a year. A lot of moms do it 3 days a week, which covers 5 of the genres. Then they do the remaining genres the following year, plus an extra. (This is how DITHOR is scheduled in the guide Preparing and following.)
Hopefully that clarified the books a little for you. Once you get everything in your hands, it's super easy to use!! If you have more questions, ask away. THe moms here are great helpers!
Kathleen