As any pub/sub messaging system, Realtime can perform one-to-many, many-to-many and one-to-one message delivery (in fact one-to-many and one-to-one is simply a particular way of using many-to-many).
For one-to-one chat you can simply define a unique channel that only the two participants know.
Another option (probably better if you intend to use push notifications) is assigning each user an unique channel (e.g. using their unique userId in your app). When user A wants to send a message uniquely to user B it simply sends a message to user's B channel.