A Co-moderator works with the primary Moderator of a game, sometimes helping process Vote Counts and Night Choices, sometimes just on backup should anything happen to prevent the Moderator from running their game (loss of Internet access, account problems, family emergency, etc). Sometimes, equal co-moderators for a single game have created a moderator account (such as London Mod) to facilitate this process.
A person who stays in the background, and only takes over moderating duties if/when the primary Moderator is temporarily or permanently away from the site, is called a Backup Moderator.
These have become more common, and more advised especially for larger and more complex games, as MafiaScum.net grows in size. Having a co-mod allows for better continuity and fewer Abandoned Games. It is now compulsory to have a Co-moderator or Backup for all Large games (whether Normal or Theme).