Combine two content types in one using search api views
You have to:

1) allow multiselect for the field;
2) implements hook_form_views_exposed_form_alter and edit the filter - replace 2 values on new value and in validate function to replace back.

Example for the simple field:

/**
 * implements hook_form_views_exposed_form_alter
 */
function MYMODULE_form_views_exposed_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if($form['#id'] == 'views-exposed-form-list-your-views'){

        $form['#validate'][] = 'MYMODULE_form_views_exposed_form_validate';

        $form['field_your_field_value']['#multiple'] = false;
        $form['field_your_field_value']['#size'] = 1;
        $options = $form['field_your_field_value']['#options'];
        unset($options[3]);
        unset($options[4]);
        $options['3+4'] = 'name of combine value';
        array_unshift($options, '- ALL -');
        $form['field_your_field_value']['#options'] = $options;
    }
}

/**
 * our validate for  form views-exposed-form-list-your-views
 */
function MYMODULE_form_views_exposed_form_validate($form, &amp;$form_state) {
    if($form['#id'] == 'views-exposed-form-list-your-views'){
        if(in_array('3+4', $form_state['input']['field_your_field_value'])){
            form_set_value($form['field_your_field_value'], array(3,4), $form_state);
        }
    }

}