Most overlapping shutter panels are rabbeted. One panel stile is notched forward and the other backwards. It's actually the same stile profile but one is forward and the other backwards. Occasionally I hear somebody complain that we used different size stiles until the notice they are identical.
An alternative to a rabbeted stile is an "Astragal". It's a small overlapping strip added to the front panel to cover the gap between panels.
Some prefer astragals and it is an option we offer for a nominal cost. However, It's a structural weak spot and often associated with plastic shutters. Our standard is rabbeted panels