It's actually impossible to come out completely fair in this kind of situation, because there are so many definitions of "fair." Some factors you might not have considered are:
- marginal cost. I've paid the entire lodging for both families before, simply because it didn't cost me any extra than a vacation with just my own family.
- financial need. One family might be better able to afford costs than the other.
- transportation costs. A family of four pays twice as much for airfare, for example.
- peer pressure. One family might usually do much less expensive vacations or meals, but compromised upward in order to accommodate the group.
- circumstantial unfairness. For example, we have to rent wheelchair-accessible vehicles when we travel, which cost about 4-5 times more to rent than a normal car.
We took a vacation with extended family this past summer and the person planning it quoted a total budget target that barely covered 2/3 of just our travel expenses. It's just really difficult to know the other person's circumstances well enough to make an unbiased judgment.
It's usually easiest to decide what's fair "on the scene." If you're grocery shopping and one person is trying to keep costs down and the other is unwilling to compromise on quality, it's usually pretty easy to decide then and there for the latter party to pick up the extra costs, but that's very difficult to predict ahead of time.
Likewise for lodging. Start at 50/50 and adjust as you feel appropriate when you get there. Maybe one family used more sleeping space, but hardly spent any time at all in the house compared to the other family. It's just easier to judge when you get there.