The PDF format was not designed to support multiple signatures. Advanced PDF viewers perform a trick when validating multiple signatures: they analyze the content that was added to see what was added after the first signature. If only a second signature was added, they show the first signature as valid.
PDF Studio version 9 and above, will validate multiple digital signatures in the same way as Adobe’s and other advanced PDF viewers.
In PDF Studio 8 and earlier, when verifying a document with 2 digital signatures that were added back to back, PDF Studio will indicate that the first digital signature is “valid with a warning”. The first signature details will indicate that the original contents was not modified, but there has been content added after the signature. Note that the signatures will show properly in Adobe’s reader and other advanced PDF viewers. This is because, when editing a PDF document that has a digital signature, PDF Studio will save the document in “update mode”, in such a way that the original content (that was signed) remains the same and new objects or changes to existing objects are added at the end of the file.