Have you ever thought about using tar? I'm not sure how it affect winzip, but I use 7z alot and what I've done as a test was to tar the two files together, then rename the tar file back to a 7z file. There is a slight overhead, about 100k, but not much. When opening as a 7z file, it will show the original pdf file. When opening as a tar file, it will show the 7z + par2 files. And when opening as a par2 file, well... it doesn't say "Appended" yet
but no parts are damaged and repair is possible as well.