@noahcoleman Strange that it’s not working for you, because it works just fine for me. Did you try putting the new attachment URL first and the original attachment URL second?
Although one key thing is that I’ve only tested with one attachment in each field, not multiple attachments in one field or the other. I will need to test later with multiple attachments.
If you can’t get it working with multiple attachments, I don’t know scripting, so I would personally do it using Make.