Hi LeAnna!

It sounds like your girls are off and running with their reading - hooray!

When doing DITHOR, dc do not need to be reading the same books, they just need to be on the same genre and be doing DITHOR the same amount of days a week.

It sounds like your older dd will probably get through half of the genres before your younger dd joins her. After that, I would just do the other half of the genres your older dd has left to do when your younger dd joins her. Then, after that, you can go back to the first half of genres you'd done, and have your younger dd read the books your older dd did for them at the start earlier, and you can choose different harder books for your older dd at that point. Let me know if this doesn't make sense! HTH!
In Christ,
Julie