You actually can have multiple accounts with the same userid. What some people will do is have a root account with a password known to only one person, and then create other id-0 accounts (sometimes called "Z accounts") for assistant admins. The nice thing about this is that you can delete one of these accounts, therefore cutting off access to someone (of course, this only works if everybody's friendly, because while they had access, they might have put in a trapdoor.)
But it's better to manage privileges with finer control. You can use the "sudo" facility to give individual accounts the ability to run any desired range of commands as root. You can do this using groups; traditionally a group named "wheel" is given the ability to run any command as root.
Try "man sudoers" (or "man sudo", if that fails) to learn more.