Currently the Real time marketing allows you add additional email fields in the Audience data for Leads however for Contact it just allows to use only one Email. So the use cases where in if there is a need to send emails to a different email id field of the Contact record then its just not possible.

In the Outbound marketing it’s a very common and popular feature. While we plan transition to Real time marketing this is a very important use case as often the journeys are configured to send emails / messages to different email ids. With Leads the Real time Marketing solution allows to configure sending SMS and emails to additional emails. the same is required for contacts.