You would need to use 2 data assist insert record behaviors.
The first insert record behavior will insert to the main contact table.
In the Session Variable section, make sure to select the contactID column and make note of the name that is set in the Store As section, this will store the contactID of the newly created record to a session variable.
in the second insert record behavior, bind the contactID column of the second table to the session created by the first insert record behavior.