[Solved] Upload library issue

... and I solved it.
Turns out allowed_types appears to be required (had the brilliant idea to comment out the allowed_types line to see what errors I got, and the same one popped up for the first upload too), so as I wanted to accept any file type in the second input, I had to set

Code:

$config['allowed_types'] = '*'

before running the second upload (ie between lines 30 and 31 in the previously pasted controller code). Other options, such as file size, don't present that issue.
A single argument had me running like a headless chicken for a few days. Dang.
Anyway, it's solved now. Sorry about the clutter.