Basically you can't, the rule is: only one of any format per book. But you can work around it, here's a couple of simple approaches,
- put the solutions pdf inside an archive format like a 7z, and add the .7z file to the book and open pdf from within the archiver (e.g. 7zip)
- copy the solutions pdf to .pdfsoln and add that to the book, and then 'tell' calibre's Open With feature to open .pdfsoln formats with your PDF reader.
BR