What is the thing with “can handle multiple signatures”?
Due to different message formats GnuPG is not always able to split a file with multiple signatures unambiguously into its parts. This error message informs you that there is something wrong with the input. The only way to have multiple signatures in a file is by using the OpenPGP format with one-pass-signature packets (which is GnuPG’s default) or the cleartext signed format.