You can query the existing users on a presence channel via the REST API: http://pusher.com/docs/server_api_guide/interact_rest_api#presence-users (bottom of page)
Note: If you give a user the same user_id
as part of the authentication process then this will stop the user being shown twice in a presence channel even if they are connected via two tabs. This is because Pusher uses the user_id
as a unique ID so won't count the subscription as a new user joining the channel.