The stores differ between a permanent duo (Simon & Garfunkel) and a temporary one where the artists might have their own profiles in the stores (Mick Jagger and David Bowie).
Are you performing as a duo? Is this a permanent duo that will last forever? Then you can add the duo as the artist in the Amuse account. If it isn't, there are a couple of prominent stores who won't accept the release.
If it is two separate artists working together on a project or two, it needs two be one artist name on the Amuse account and the other artist will be added as a primary artist as well when the upload is made. The release will be seen on both profiles in the stores.
This is how you do it when you have separate side projects and are not always a duo:
You start an account with only one artist name, make an upload and add the second person (not on the account) as a featured artist. Contact firstname.lastname@example.org and we will then add the featured artist as a primary artist as it can't be done in the app (yet).
If both primary artists want access to the account in the app, simply sign up with a Google Account that you both have access to, or by signing up with email and sharing the password.