I have tried:
cat file1.ipynb file2.ipynb > filecomplete.ipynb
since the notebooks are simply json files, but this gives me the error
Unreadable Notebook: Notebook does not appear to be JSON: '{\n "metadata": {'
I think these must be valid json files because file1 and file2 each load individually into nbviewer, and so I am not entirely sure what I am doing wrong.