We have Acrobat Pro 9, 10, and 11 being used by different users on Windows PCs. We have created a document with 2 digital signatures, and need to lock it for editing after the second person applies their digital signature. How do we do that?

We have Acrobat Pro 9, 10, and 11 being used by different users on Windows PCs.  We have created a document with 2 digital signatures, and need to lock it for editing after the second person applies their digital signature.  How do we do that?