By default, Joomla allows a user to be logged in only once. If they (or someone else) logs in at the same time from another computer, the original user will be logged out. This is for security, but sometimes you want to allow this (for a demo site, for example).
You can set up Joomla to do this, but it requires a simple change to one of the core files.
You will need to edit includes/version.php. Look for this code:
Change the $SITE to
This will allow multiple user logins with only one account. By default Joomla! allows only one active session per account as a security feature.
You should probably also change $RESTRICT to:
This change disables those logging in, both frontend and backend from changing user details - like password and username.