Can I WhatsApp between MultiLine users in the app?

No. The other party is required to have an active account on the WhatsApp messaging app.

The WhatsApp messaging connector only allows you to send WhatsApp messages to WhatsApp users from within MultiLine. It is essentially a bridge from MultiLine to a WhatsApp business account. That business account can only send and receive messages from WhatsApp users, not from MultiLine users.

Note: the WhatsApp business account is associated with a Twilio number rather than your MultiLine number. Any WhatsApp messages to that Twilio number from a WhatsApp user will be received by the MultiLine app. If someone attempts to WhatsApp your MultiLine number, it will not work, because your MultiLine number is not associated with the WhatsApp business account.