Multiple registrations isn't designed for multiple users to share an extension in the manner you're describing. A typical use case would be for a single person to have a desk phone and one or more additional endpoints such as a soft phone, cordless, or perhaps a remote office desk phone.
If you need a call to ring multiple phones for multiple users to answer, Ring groups, Call center, or Call groups are the ideal way to route calls.
If a certain extension needs the option to be picked up from other phones, or alert other phones in case the primary user isn't at their desk, Follow-me or call pickup are viable options.