HI Christophe,
I checked with R&D and they indicated you are correct about copying the folders:
In this case you must copy the entire folder C:\Program Files\Astea Publisher 15.0 from one server to another as they must be the same so the client will get updates from both servers with same manifest.
Please backup the folder C:\Program Files\Astea Publisher 15.0 on the second publisher server and copy the folder C:\Program Files\Astea Publisher 15.0 from the first publisher server to the second publisher server.
Otherwise, there may be issues getting updates down to the browsers for example.