We did 17 hours with a 11 month old.
We started at 5PM drove 3 hours with one in the backseat singing songs, playing with small toys, drawing, etc. we then stopped for dinner, got DC into PJs did note time routine (book, bottle, brush) and hit the road hard. The deal was 3 hours each one while the other slept so DH did the first stretch till dinner I did the second stretch. He was snoring so I just kept driving until he woke up. It was 11 hour later LOL
My suggestion is do night time routine and put baby to sleep in the car. Then drive 4 hours each one. You can even plan a stop and you both take turns staying in the car.