If you're manually performing the search with the ORM; $list->filterAny()
If you're using fulltextsearchable, then ... things are a little more difficult.
If you've manually applied fulltext filters and are manually making the search, then you have control over what input goes to what field. Simply use the same one twice.
->filter(['Title'=>$Search->value(),'Description'=>$Search->Value()])
(or something like that).