Ably is a pub/sub system and it broadcasts messages to channels. So, in order to send a message, you publish to a channel. The only exception is when using push messaging - the push admin API has a method that enables a specific push message to be sent to a specific registered device.
If you wish to send a message between two users, we recommend you create a channel specifically for this communication. For example, if you wish to have a "broadcast" operation from admin1 to all the devices of user2, you can have all the devices of user2 attached to a specific channel upon which admin1 publishes messages. This will effectively allow communication between two specific users, even allowing for it to work on all of the user's devices.