How can I put two or more signatures in a message?
Although there are a few variations for putting multiple signatures in an update, please note that there is yet no reported way to consistently do this via mailer interfaces. So, again the most straightforward way is to do this from the command line. For the first signature, just sign the message as explained in the previous question. For the consecutive signatures, sign the resulting .asc files from the last signing. Send the final resulting file to firstname.lastname@example.org, which will carry all authentications.