modify-field-data-before-submit.js

// Create a new object for custom validation of a custom field.
var mySubmitController = Marionette.Object.extend( {
  
  initialize: function() {
    this.listenTo( Backbone.Radio.channel( 'field-4', 'change:field', this.nameChange ) );
  },

  nameChange: function( el, model ) {
    // Split your model value
    var value = model.get( 'value' );
    var firstNameValue = ''// First name logic
    var lastNameValue = ''// Last name logic
    // Get our first name model and update the value where 5 is the field ID
    var firstNameModel = Backbone.Radio.channel( 'fields' ).request( 'get:field', 5 );
    firstNameModel.set( 'value', firstNameValue );
    // Get our last name model and update the value where 6 is the field ID
    var lastNameModel = Backbone.Radio.channel( 'fields' ).request( 'get:field', 6 );
    lastNameModel.set( 'value', lastNameValue );
  },

});

// On Document Ready...
jQuery( document ).ready( function( $ ) {

    // Instantiate our custom field's controller, defined above.
    new mySubmitController();
});