We've done this a few times, and I'll tell you what worked for our family. YMMV, of course.
We've found that there IS such a thing as too much togetherness. It's important for everyone to have some space as a family. We found that we did enough advance planning to have everyone in the same park on the same day, and have dinner together. That way, the late sleepers wouldn't irritate the rope-droppers. We had a wide gap in children's ages--if I went now with the same crowd, I'd consider letting teens roam by themselves (with cell phones and scheduled meet-up points).
What worked for us was, we had one person from each family go into a room to plan/mediate. In our case, MIL didn't care, she was along for the ride. Her only restaurant choice was Coral Reef, so we incorporated that. BIL and I were the two designated planners--this worked well because we respected each other, got along, and weren't siblings. Also, we tried real hard to see the other's point of view. We would each bring our tentative itineraries and restaurant choices. Then, we'd sit in a room and hash out our schedule/ADRs. No one else was permitted to comment. When we were done, we each presented them to our families, got feedback, and then came back for our final lists. This probably sounds more militant than it really was--everyone got at least close to what they wanted--nobody felt like they were getting the shaft. And it wasn't just one person taking over, KWIM?
Now, with my own family, I have 4 kids spanning 11 years. Even that can get dicey, believe me! When we vacation, I let each kid pick one "must do" restaurant, and three "must do" rides. Obviously, we do a lot more than just that, but each one feels that they, at least, got what they had asked for. I find that, for example, DD14 is much more tolerant of waiting to see Ariel when she recognizes that DD6 waited with Mom while she did Expedition Everest.