I am hoping to get some clarification on the ‘frame within a frame’ detail for double-stud wall door hanging. 

I am referencing the detail found in the GBA library (and other similar).

1) In the door head detail, there are two horizontal 2x’s below the header. I understand one, given the 2 ply (3″ wide) header; why two?

2) How is the frame (within the frame) attached, especially if one of the 2x’s lands in the plywood bridge of the buck? Toe nailing into plywood doesn’t seem like enough. Also, do folks glue the foam in the frame sandwich to make it all a rigid unit? 

3) To confirm: Functionally speaking, an extra frame is only needed on the hinge side. The frame is added to the striker side and the head simply to create a symmetrical appearance / for trim. Correct?

Thanks for any guidance,
Tyler