While I think they could possibly switch over to Pens pregame at 6:30, I don't think they would unless the game was already decided and even then I would think they would wait till its over. That said, if this game went past 7, and with Duke throwing 60 passes probably, you never know, they definitely would switch to the Pens game at 7 even if the Pitt game wasn't over.
You mention the ESPN app though but that wouldn't help. These Fox Sports South RSN syndicated games are always blacked out locally. I remember watching UVa vs Syracuse last year because we had both teams coming up and it went to OT and they switched to a Pens or Pirate game so I went to the app to watch it and it was blacked out even though Root took it off.
Also, many times Root does not broadcast these games live due to Pens, Pirates, or WVU games so they will air the ACC FSS game around midnight. I have tried to watch them live on the app but they are still blacked out. I remember trying to watch UNC/Wake last year which was like that and it was blacked out locally even though it wasn't on live.