It seems that the user-selector component cannot be used more than once in a form. They both get given a name attribute of user-selector-renamed, but if you try to change the value of the second then it refreshes the page.
I tried manually setting the name property of each component (with no usernames property), and both are now useable, though I don’t think it updates the model/controller. The name attributes of the elements are now undefined-renamed and <(unknown):ember53>-renamed.
I’d find it very useful for a plugin to be able to have multiple user-selectors in one form. Could the component be modified to more safely allow this? It should also be possible to have labels associated with the input elements. (I don’t think this happens currently even if you only have one.)