How to export data from several documents that don't share the same document definition in the same export?
A Document set should be used.
When you create a Document set you should specify what Document Definitions should be included.
Once the Document set is created, the export that is specified in its settings will be applied to all the documents that are combined within it.
If there is an export defined in the Document Definitions that are inside of the Document set, additional exports will be performed (there will be an export for each Document Definition that has export configured and export for all the documents within the Document set)