Freron MailMate

I'd like to be able to sign a message simultaneously with both S/MIME and GPG, so that those recipients with a S/MIME-enabled MUA are able to verify the signature, as are those with a GPG based MUA. I'd like recipients to be able to verify the message unconditionally. This could be done in a nested way (for example, one signature includes validation of the other one) or in a parallel way (both signature types validate the content). However if this is done, it's important that the message, along with all of its ordinary attachments, be included in the signature calculation. And the presence of an additional attachment (such as the other signature) shouldn't affect the validity of the other signature.