function forms_merge_two_fields( $update_data, $user_id, $contact_id, $form_id ) {

    $update_data['StreetAddress1'] = $update_data['_CustomField1'] . ' ' . $update_data['_CustomField2'];

    return $update_data;

}

add_filter( 'wpf_forms_pre_submission', 'forms_merge_two_fields', 10, 4 );