There seem to be two different types that people refer to:

1) Running one or more account at the same time, sometimes having supportive conversations. This seems to require logging in and out and in a couple of instances people doing that have been caught out because they forgot to log out of their account and into the other one:

2) The same individual having more than one account running (usually a sleeper account but sometimes a new one). These people are used to being banned or outed so they materialise in another guise usually swearing blind they are "longtime lurkers" rather than an old face with a new broom.