Can WebSync send messages from one user to some other specific user?

Absolutely, the recommended approach is to have particular users subscribe to unique (personal) channels. For example you might get your own personal channel using some predictable channel identifier like my.name@my.organization.com. When you sign into your app you would automatically be subscribed to this personal channel. When other users want to direct message you they will simply publish to this channel. Note, they should not be subscribed to your personal channel, they simply publish to it. Also, from the server side you can publish to specific channels as needed.