My extended family likes to travel around together a lot. I agree with everyone that communication with the other family about wants, expectations and spending habits is important.
I think the first big question is how close you want to tour with each other. Will you just meet up for a ride and/or meal or two or do you want to do most of thing things together. It can work that you spend the entire time together but that involves a lot of communication and compromising between the families. And it has to be what both families want.
Some important things to talk about are:
-Park times: Sleeping in, rope drop, late nights, nap times, bedtimes.
-Splitting up: and not just by family (which is also important to know if they or you want to do some family alone time) Are you will to split up by taking one set of kids to one ride while the others do something else with the other adults or will the parents want to stay with their kids the entire time.
-Ride types: is there anyone who won't want to ride a certain type of ride. One family's must ride attraction may not be on the list for the other family.
-Meals: counter service, sit down, or bringing your own food. Do you want to eat all your meals together or do you just want to do one a day
-Snacks (and souvenirs): One family gets ice cream then the kids of the other family will want some as well but it might not be in their budget. Or one family gives their kids $50 for souvenirs while the other only gives $20. Neither family is right or wrong but by knowing that one won't have the money for an extra treat you can get it while your touring alone and save the awkward moment and kids tears, or you can manage your kids expectations better so they won't be upset if their friend can get the bigger toy. (and I'm not saying you have to know each family wants to spend x amount. But just knowing if they or you are going all out or don't want to be spending to much extra can go a long way)