Did some research on this. Prep hasn't had a two way starter at all during this three-peat. They only had 1 in 2016 (Richard Carr at receiver/safety), had four in 2014 (Swift, Reid, Zaccheus in the secondary and Jake Strain at DE/TE), and two in 2013 (Reid and Steve Robinson at OT/DT). The only other teams with only 2 two-way starters on Big School champs are McKeesport in 1994 and Parkland in 2002.
Three is a little more common (PCC '88, CB West '98 and '99, Neshaminy '01) and so is four (CB West '97, PCC '07, PCC '15, Pine-Richland '17). But historically, most teams have had nearly half of their lineup going both ways to win a title at the highest classification.
5 Two Way Starters: Upper St. Clair '89, North Allegheny '90, CB West '91, Downingtown '96, Central Dauphin '11
6 Two Way Starters: St. Joseph's Prep '03, PCC '04, McKeesport '05, Liberty '08, North Allegheny '10
7 Two Way Starters: Cumberland Valley '92, Erie Cathedral Prep '00, North Penn '03, LaSalle '09, North Allegheny '12
8 Two Way Starters: Penn Hills '95, Upper St. Clair '06,
10 Two Way Starters: North Hills '93
What's impressive about Prep is how disciplined they are about not using their stars going both ways. In Harrison, Hagans, Cooper, Trotter, Harris, Nelson, Lombardi, Burrell are all three and four star recruits who would be exceptional on the other side of the ball. Not a lot of teams have done that. Even that McKeesport '94 team that stayed pretty two-platoon - Brandon Short was playing offense and defense. Jon Ritchie was on both sides of the ball for Cumberland Valley '92. CB West had Picciotti going both ways in '98. Downingtown had Arlen Harris and Dan Ellis on offense and defense. Liberty had four guys who signed NFL contracts in '08, all four played both ways. Cathedral Prep '00 had 7 FBS players, all 7 went both ways. The depth that SJP has to not do that is just incredible.