The basic stars are Shaun Cassidy and Parker Stevenson as Joe and Frank Hardy (respectively), and Pamela Sue Martin as Nancy Drew. In general, the show does a Hardy Boys mystery and a Nancy Drew mystery. The two seem to not cross paths until the beginning of series 2 (which I have yet to watch). The Hardy Boys episodes are often either very Scooby-Doo, Where Are You? or Johnny Quest flavored, dealing with pretend ghosts, aliens or espionage, and Nancy Drew deals primarily with thefts, though they often have a "haunted" element.
As for the structure, Joe and Frank work for their father, investigator Fenton Hardy, and have some sidekicks. Callie Shaw (Lisa Eilbacher) shows up a lot. Nancy works on her own, but is often assisted/guided by her father, lawyer Carson Drew. Nancy has two full-time sidekicks in George Fayne (Jean Rassey) and Ned Nickerson (George O'Hanlon Jr.).