Additional accounts created with UID 0 are still root accounts, albeit with a different name and password. SSHd therefore (correctly) restricts logins as any alias to root exactly as it would restrict logins as root itself.
A more-ideal solution would be to create multiple non-root accounts with root-level access granted through
sudo on a per-command basis. This not only increases the granularity of control, but also gives you better logging of administrative behavior.
An alternative method of granting access to multiple people for the same account through ssh is by using ssh keys. The sshd configuration option
PermitRootLogin without-password goes hand-in-hand with this mechanism, allowing root login only using mechaisms that don't involve providing the password (ssh keys being the most common of these). The primary reason for doing this is to prevent brute-force password attacks against root.