After looking around and not being a Shotgun code capable Admin, I think that to use multi storage is not possible without a Schema.yml
The storage configuration in the interface is only to stipulate 2 different location, BUT that in any case allows us, simple human beings to leverage that attribute without a Schema.yml specifying with folders which storage to use.
It is a bit frustrating to see an attribute available there but being totally useless if no custom Schema file created.
It is frustrating because you don’t really need to do it via the interface, you can do it by simply altering the file roots.yml with text editor.
So back to square one.
I think Shotgun team should think of a Custom Creator Script you can run at project creation and take the Admin (Simple human being) through those step and creating a Custom Schema.yml on the fly. Like:
1 - Do you want multi storage enabled ?
2 - Which Step would you like to attach to Storage A or B
The difficulty of Shotgun right now is that it is pretty much an expensive Google Sheet if not Customized. Not every small boutique, studio can afford a Company to tweak and Custom your SG pipeline, when they charge average 5 to 10K for starting point and put a 200 hours of maintenance behind that OR hire a full time shotgun developer. The entire price tag of the consulting is most of the time higher than the benefice of the project after completion. An nowadays with remote work etc, we can really use quite a lot of artists for very specific tasks, which at the end benefices from pipeline configuration like the one you or I are looking for.