If multiple signatures are to be integrated into a PDF, this is done by means of incremental PDF updates (explicitly not by adding multiple SignerInfo structures to a single integrated CMS signature container!):
The cryptographic verification of these signatures merely guarantees that the byte range signed by the respective signature has not been tampered with. If does not guarantee that PDF objects defined in that range have not been replaced with other ones in any newer revision.
Checking in what way the additions in later revisions change the appearance of the content signed by an earlier signature is a separate verification task which is not trivial.