Right now, you have to show at least one emergency contact field which only has text input options. If you click +Add Another Emergency Phone Contact you then get the option to choose from known contacts or add a new contact (the same text input option as above).
Let's say someone wants to add 2 contacts, at least one of which is an existing relationship. They see the first entry which is required, so they'll input text, but when they go to add the 2nd contact they'll see the info from the one they just typed in the known-contacts list since it's already an existing relationship!