At Proton, any individual user account can be associated with multiple artist profiles.
To add a new artist profile to an existing user account, just follow these steps:
Create the new artist profile in SoundSystem if you haven't already.
Add the artist to your track, as an original artist, remix artist, etc.
In the royalty area, click "Add a Member" next to the new artist name.
In the window that opens, look for the text "Add a person to this artist?" -- click the menu next to that text, then search for the existing user's real first name and last name in the database. Select their name, and then click "Submit" next to the menu.
The area below that menu is to create a brand new account for users not already in the system. If you try to add an existing user to the database in that area, you may get the following error: "Oops! A person already exists with that email address." -- that means they're already in the system, so follow the steps above to add them to the artist profile.