AcroForm not visible when merging documents

Question

I'm trying to merge documents side by side with PDFBox, using the following code:

function void generateSideBySidePDF() {
    File pdf1File = new File(FILE1_PATH);
    File pdf2File = new File(FILE2_PATH);
    File outPdfFile = new File(OUTFILE_PATH);
    PDDocument pdf1 = null;
    PDDocument pdf2 = null;
    PDDocument outPdf = null;
    try {
        pdf1 = PDDocument.load(pdf1File);
        pdf2 = PDDocument.load(pdf2File);
        outPdf = new PDDocument();

        // Create output PDF frame
        PDRectangle pdf1Frame = pdf1.getPage(0).getCropBox();
        PDRectangle pdf2Frame = pdf2.getPage(0).getCropBox();
        PDRectangle outPdfFrame = new PDRectangle(pdf1Frame.getWidth()+pdf2Frame.getWidth(), Math.max(pdf1Frame.getHeight(), pdf2Frame.getHeight()));

        // Create output page with calculated frame and add it to the document
        COSDictionary dict = new COSDictionary();
        dict.setItem(COSName.TYPE, COSName.PAGE);
        dict.setItem(COSName.MEDIA_BOX, outPdfFrame);
        dict.setItem(COSName.CROP_BOX, outPdfFrame);
        dict.setItem(COSName.ART_BOX, outPdfFrame);
        PDPage outPdfPage = new PDPage(dict);
        outPdf.addPage(outPdfPage);

        // Source PDF pages has to be imported as form XObjects to be able to insert them at a specific point in the output page
        LayerUtility layerUtility = new LayerUtility(outPdf);
        PDFormXObject formPdf1 = layerUtility.importPageAsForm(pdf1, 0);
        PDFormXObject formPdf2 = layerUtility.importPageAsForm(pdf2, 0);

        // Add form objects to output page
        AffineTransform afLeft = new AffineTransform();
        layerUtility.appendFormAsLayer(outPdfPage, formPdf1, afLeft, "left");
        AffineTransform afRight = AffineTransform.getTranslateInstance(pdf1Frame.getWidth(), 0.0);
        layerUtility.appendFormAsLayer(outPdfPage, formPdf2, afRight, "right");

        outPdf.save(outPdfFile);
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if (pdf1 != null) pdf1.close();
            if (pdf2 != null) pdf2.close();
            if (outPdf != null) outPdf.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

However the form fields contained in the original documents are not displayed in the final PDF. I also tried to set the acroform on the final document, doing:

outDoc.getDocumentCatalog().setAcroForm(acroForm); 

but it doesn't work.