How to map two arrays to one target with repeating children

Summary:

Does anyone know if it is possible to join two json files together in OIC? I have to report the ID and projectID in one response payload back to target system, so I am using write and stage files to build the structure, but I have to do so from two different arrays in the payload. I tried appending a write file to the first write file, but it just overwrote the existing json instead and gave me a warning that json might be incompatible for a second write. 

Content (please ensure you mask any confidential information):

 (both "added" and "changed" are children of the "financialAccounts" array, if that matters)