Here is C# code that demonstrates how to add 2 digital signatures to a PDF document: 
using System.Drawing;
using System.Security.Cryptography.X509Certificates;
using Vintasoft.Imaging.Pdf;

/// <summary>
/// Adds two digital signatures with text appearance to a PDF page.
/// </summary>
public class PdfPageDigitalSignatureHelper_Multi
{
    public static void Run()
    {
        // open PDF document
        using (PdfDocument document = new PdfDocument("TestDocumentToSign.pdf"))
        {
            // get the X509 certificate from .pfx-file
            using (X509Certificate2 certificate = new X509Certificate2("TestCertificate.pfx", "test"))
            {
                // create a helper that alows to add digital signature to a PDF document
                PdfPageDigitalSignatureHelper digitalSignatureHelper =
                    new PdfPageDigitalSignatureHelper(certificate, new RectangleF(20, 20, 250, 100));

                // set the reason for signing
                digitalSignatureHelper.SigningReason = "Approve document";

                // set the signature appearance text
                digitalSignatureHelper.SignatureText = string.Format("Digitally signed by\n{0}, page 1",
                    digitalSignatureHelper.SignerName);

                // add signature to first page
                digitalSignatureHelper.AddSignature(document, 0);

                // set the signature appearance text
                digitalSignatureHelper.SignatureText = string.Format("Digitally signed by\n{0}, page 2",
                    digitalSignatureHelper.SignerName);

                // add signature to second page
                digitalSignatureHelper.AddSignature(document, 1);
            }

            // sign PDF document and save PDF document to the output file
            document.SaveChanges("TestDocumentToSign-signed.pdf");
        }
    }
}