Preheat oven to 400 F.
Combine the sugar and cinnamon in a small bowl.
Roll out the two puff pastry sheets into 12" x 12" squares.
Cut each sheet into 4 3" wide strips.
Cut each of the strips in half. You will now have 12 rectangles that are 3" x 6".
Divide the apple pie filling among 8 of the rectangles. Just mound it up a bit down the center but don't get it too close to the edges. You need to keep a little bit 'clean' so that the edges will seal.
Using a sharp knife, cut 4 diagonal slits in the remaining 8 rectangles. Don't get them too close to the edges as you don't want the dough to split.
Place the rectangles with the slits in them on top of the apple pie-topped ones.
Using a fork, press down along the seams to seal the pastries.
Combine the egg white and water in a small bowl or glass using a fork. Brush over the tops of the pastries.
Sprinkle the pastries with the cinnamon sugar mix.
Transfer to a baking sheet lined with parchment paper or with a silicone mat. I used a large spatula to move them. It can seem a bit daunting but if you're careful they'll make the trip just fine. You might have to nudge them around a bit to get them back to perfect looking rectangles.
Bake for 15-20 minutes or until golden brown.
Remove to a wire rack to cool slightly before serving.