Cascade GPG signing

I'm trying to solve problem with secure mailing lists

the idea is:

Person A signs (or cipher) mail and sends to groupmail. On server this mail is verified against correct key, signed (ciphered) again and send to recipients (each mail separately).

the recipients first verifies (deciphers) against groupmail key, than against Person A key.

I made python script to do this, and stack on cascading. The problem is that enigmail can only recognize the envelope sign, and no inner sign. Maybe its a problem of engimail, but I think, that I should add some headers to email to suggest that this is twice signed.

Now this looks like: (header OpenPGP is created by myself, the hash keys are cutted)

(...)
OpenPGP: id=A028D47A; id=09813F09


-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

- -----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

TEST
- - -- 
Tomasz Brzezina
- -----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.12 (MingW32)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/

iQEcBAEBAgAGBQJNZPioAAoJEGDoB6ygKNR6RKYH/A7gNGf+C5TERWDIVQUmykfs
Y+XAv2u3b9aK48T+WRWsS+mwt0fRhMy1TFjLua2Xta5FfBFzZdXFH9mobSAOQFw=
=/YoZ
- -----END PGP SIGNATURE-----
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.10 (GNU/Linux)

iEYEARECAAYFAk1k+KQACgkQNgqC2gmBPwnWjQCfSTbWixnGXMEa3mjiUhaOMttB
=iZWb
-----END PGP SIGNATURE-----

The result of mail is:

********* *BEGINNING OF SIGNED/CIPHERED PART* *********

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1


- --
Tomasz Brzezina
Sekretarz Generalny UPR
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.12 (MingW32)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/

iQEcBAEBAgAGBQJNZPioAAoJEGDoB6ygKNR6RKYH/A7gNGf+C5TERWDIVQUmykfs
Y+XAv2u3b9aK48T+WRWsS+mwt0fRhMy1TFjLua2Xta5FfBFzZdXFH9mobSAOQFw=
=/YoZ
-----END PGP SIGNATURE-----


********** *END OF CIPHERED/SIGNED PART* **********

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.10 (GNU/Linux)

iEYEARECAAYFAk1k+KQACgkQNgqC2gmBPwnWjQCfSTbWixnGXMEa3mjiUhaOMttB
xtAAn2PoG+HhPg2RO1YoqKObFevZErHv
=iZWb
-----END PGP SIGNATURE-----

As posted by Helmut Grohne above, MIME may help your problem. Yes, engimail does support MIME.