XAdES internally detached signature

I want to create a file that contains multiple independent signatures referencing the same data. I want the data to be stored in the output XML file, as a sibling to ds:Signature elements, like below (only with multiple ds:Signature elements):

<?xml version="1.0" encoding="UTF-8"?>
<internally-detached>
  <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
    <ds:SignedInfo>
      <ds:CanonicalizationMethod 
         Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
      <ds:SignatureMethod 
         Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
      <ds:Reference URI="#data">
        <ds:DigestMethod 
          Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
        <ds:DigestValue/>
      </ds:Reference>
    </ds:SignedInfo>
    <ds:SignatureValue/>
  </ds:Signature>

  <document Id="data">
     <title>title</title>
     <author>writer</author>
     <date>today</date>
     <content>
        <para>First paragraph</para>
        <para>Second paragraph</para>
     </content>
  </document>
</internally-detached>

Source: https://msdn.microsoft.com/en-us/library/ms759193(v=vs.85).aspx (I am aware that the site does not mention XAdES standard).

I tried to apply this to a valid enveloping XAdES signature and putting the referenced ds:Object node outside the ds:Signature element, but the verification software returns an error (referenced node was not found), and the xades4j verification module fails after encountering the </internally-detached> tag.

Is an internally detached signature possible with XAdES? If so, how can I achieve that?

I know that multiple signatures are possible when the data is stored in a separate file: each signature's SignedProperties element must contain SignedDataObjectProperties with ObjectReference, and the URI in SignedInfo's Reference element should point to the file.

That's certainly possible with XAdES signatures, since they are also "regular" XML signatures.

I'm not sure what you mean by "xades4j doesn't even allow a root node other than ds:Signature"; your objective can be easily achieved.

Assuming you have the base XML document without any signatures (either from a file or created using DOM APIs):

<internally-detached>
  <document Id="data">
     <title>title</title>
     <author>writer</author>
     <date>today</date>
     <content>
        <para>First paragraph</para>
        <para>Second paragraph</para>
     </content>
  </document>
</internally-detached>

Having this and, lets say, 2 differently configured instances of XadesSigner, you can do something like:

XadesSigner signer1 = ...;
XadesSigner signer2 = ...;
Document doc = ...; // your base XML
Element signaturesParent = doc.getDocumentElement();

SignedDataObjects objs = new SignedDataObjects(new DataObjectReference("#data"));
signer1.sign(obj, signaturesParent);
signer2.sign(obj, signaturesParent);

Just beware that you need to define the "data" element's Id attribute as its XML ID.

This should produce a document similar to:

<internally-detached>
  <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
  ...
  </ds:Signature>
  <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
  ...
  </ds:Signature>
  <document Id="data">
     <title>title</title>
     <author>writer</author>
     <date>today</date>
     <content>
        <para>First paragraph</para>
        <para>Second paragraph</para>
     </content>
  </document>
</internally-detached>