In signature CAdES it's possible create multiple signature sequential or parallel.
Signature parallel: in case of contract, when more people have to sign the same document. Each people sign the same document, and each signature is added to document.
Signature sequential: where person sign the entire document (signature including) previously signed from another person.
My question is: in signature PAdES it's possible to create signature parallel? For signature PAdES we use itext.
No, it is not possible, PAdES signatures are always sequential. Each signature is made on the current version of the document
Take a look to the ETSI TS 102 778-1 PDF Advanced Electronic Signature Profiles