Keep in mind, you're probably never going to a see a multi-box campaign. Looking at Descent and Mansions of Madness and how things have worked for IA in the pre-digital age all the campaigns/stories are Core +1 expansion. They do this because they don't want to release something that people wont get or can't play because they don't have the other box needed.
I could be wrong, and it might happen someday, but don't be disappointed if further releases don't combine multiple expansions.