Merging 2 arrays into single array

nickhumble - my apologies - I missed your original response to my question.

If there are two input documents that you want to combine to a single document, I would recommend using the Gate snap in this case.  Then follow with a Mapper snap with the following expression:

The jsonPath is a powerful function that allows you to rip through a JSON structure looking for specific sub-elements easily and return the results as an array.  This specific syntax tells jsonPath to return all elements buried under the top three arrays. 

Note that the Gate snap has some considerations when processing larger volumes of data, but for this case, it works very nicely.  Also, the jsonPath documentation has some external links for additional details of how you can work with it along with some good examples.

Hope this helps!