By Denny Dyroff, Entertainment Editor, The Times You can enjoy a two-for-one treat if you visit Longwood Gardens (1001 Longwood Road, Kennett Square 610-388-1000, www.longwoodgardens.org) on August 18. The featured attraction will be a concert by Kennett Symphony of Chester County at 7:30 p.m. in Longwood’s Open Air Theater. Visitors to the site will also be able to check out Longwood’s amazing...