Road trips to Disney can be magical all by themselves!
I can't offer specific advice about your route, but I have a tangential suggestion about the road trip itself: in planning entertainment for the girls along the way, don't overlook the "old-fashioned" kinds of in-car entertainment. A lap desk and a fun binder for each girl with copies of coloring pages, word search games, license plate game sheets, mazes, stickers, Mad Libs, "Question book"-style interview questions for family discussion, a map of your driving route, etc. can provide many hours of fun. Also, since they haven't been to Disney before, you can add some Disney park maps, and copies of Birnbaum's for Kids (or Guide to the Magic for Kids, which is especially great for younger children -
http://www.guidetothemagic.com/shopping.htm) to give them a better idea of what they're "in for" and help build their excitement. To keep things interesting, you could also distribute care packages with small Disney-related gifts (e.g., new sunglasses or hats, Disney music CDs for the car such as the "Frozen" soundtrack or the Walt Disney World official album, autograph books and pens, card games, favorite snacks), to be opened at various milestones along your route.
I think your decision to split the trip up into 2 days is wise, and I respectfully disagree with anyone who suggests trying to drive straight through. I don't mean to be a downer, and I know many, many people do it successfully (my Dad included, who used to do that when we made the 20-hour trip to WDW from upstate NY when I was growing up), but I personally know two different families who had catastrophic motor vehicle accidents during long car trips -- in both cases, husbands and wives were switching off driving/sleeping overnight, and the driver simply nodded off and went off the road. In one case, their spouse was killed, and in the other, was paralyzed from the neck down. Even if you "lose" a night's worth of driving time, it's worth it to be well-rested and to have two pairs of eyes on the road at all times.