SELECT * FROM `table_name` where timestamp_col1 AND timestamp_col2 BETWEEN
'2018-07-23 11:45:25' and '2018-07-23 14:45:25'

$this->getEntity()
     ->whereBetween('timestamp_col1', [$start, $end])
     ->orWhereBetween('timestamp_col2', [$start, $end])
     ->get();