Currently it’'s not possible to send a message to a bare JID and let the server send the message to all the connected resources with the same priority. This feature is planned for Messenger 2.2 under this issue: JM-14. You can vote on the issue to raise its priority.
If you need to send messages to a group of users then you can try using the broadcast plugin. Other option, that requires a little of development, is to write a plugin that provides a broadcast service. So instead of sending the message to the bare JID of the user you will send the message to the new broadcast service and let the new component broadcast the message to all the connected user resources.