[NTG-context] (off-topic) concatenating signed PDF documents

Dear list, I have a question for users with more knowledge on digitally-signed PDF documents. Is it possible to concatenate PDF documents (each signed by a different certificate [by a different person]) into a single PDF document without invalidating any signature? I tried to join all PDF files with pdftk, but digital signatures in the resulting file were invalid. I guess that I’m trying something impossible, but I’ll really appreciate the confirmation from somebody with experience in digital-signed PDF documents. Many thanks for your help, Pablo -- http://www.ousia.tk