We should support the following usecase in Kleopatra:

A document has to be signed (OpenPGP) by two (or possibly more) different persons.
In order for a one-click verification by the recipient of the document we need one signature file with both signatures. And it needs to have only the expected document name in it's filename before the ".sig".

At the moment it is only possible to concatenate signature files on the cli, which is not very user friendly.

Edit (2024-06-17):
The concatenation is trivial, because the signatures are binary they can simply be concatenated.
Add an extra button "Append" in the overwrite dialog. Before concatenating it might be useful to first use gpgme_op_identify to assert that the first is really a signature. The validation of the first signature is not necessary at that point.

Old:
Would it be possible - if we find a signature already next to a document - to ask if we want to replace the signature or to add a new one in the same file?
For this we should check first, if the first signature is valid. If invalid we would offer replacement only.