I would invest in a portable dvd player. That will save you!! Also coloring books, childrens books, bring a couple there favorite toys...and hope for some nap time...bring a couple pillows....bring some snacks that they like. Make some frequent stops to let them stretch there legs so they dont get cranky.
Lot's of music for the three of you to sing along with, bring stuff that they both like. Also the portable DVD player is a good idea as well, just not for the whole time. Have a bunch of car activities, games (choose two letter and the first one to find those two letters right next to eachother chooses the next 2 letters).
We drove to FL from CT in a huge rented van with 10 people packed inside, including little kids. We had a portable DVD player w back up batteries. Lots of paper/ coloring books, crayons and markers. Travel board games and lots of snacks. We also played fun car games to pass the time...
Bring books on tape/cd choose a small series to be able to listen to multiple stories. When I went from Maine to Arizona when i was little we listened to Harry Potter. I would try the local library to get them for free. I also recommend tiny goody bags that could be handed out along the trip. Just from the perspective of someone who grew up driving that route pretty much once a year.
My 4 year old loves having the car DVD player, but on a long trip like this, you have time to bond with your girls. I remember my childhood car trips being mostly boring, but I also remember talking with my Mom and Dad, playing I Spy or the Alphabet Game (using signs and licence plates, go A through Z). Coloring books were boring, but music was always nice. Take the DVD player as a backup, the make the trip as memorable as the destination.