Yes, every author has to register first to be able to submit the
abstracts or edit the abstract as co-author. So, you should include all
the contact information in the registration form and the rest in the
abstract submission form. That way every co-author can fill his/her
information on her own by logging in to the dashboard using the
credentials that is sent to these co-authors when the main presenter
first adds them during abstract submission.
So, what you should do is separate the fields into two forms: author registration form and abstract submission form with their respective fields.