Not sure if this is a roofing question or structural, so I will post in both.
Roof has a sag of about 1.5" and 3 or 4 feet wide
Attic reveals a rafter placed between two sets of doubled up trusses, rafter is the low point of the sag. My report already refers to an engineer, because whatever was intended does not appear to be working :-), but my question is, why would anyone do this? Home built 2006.
Trusses sheathed with 7/16-inch OSB are typically 24" on center. That looks like about 48". If they installed the rafters when the sheathing was already sagging without pushing the sheathing back up into place, it would look just like that.
Why they did it would be a guess. Looks like they were short a truss. Maybe they broke one, mis-ordered, or extended the floor plan.
I don’t see any reason to double up on the trusses. so it’s a mystery to me. Also, there’s vents visible in the exterior photo but not in the one in the attic. Is this the same area?
Based on where your picture is taken, are you in the opening to the attic? Did they not place a truss there due to the attic access and that’s why they doubled up the trusses on either side?
During framing, they install the access hole between trusses, since the trusses are installed first, and that’s why the typical modern access size is 22.5"x30".
Also, doubling the trusses on each side wouldn’t do anything to reinforce the poorly-supported sheathing unless they were supposed to install some kind of framing that would be supported by the trusses.
Looks like the trusses were spaced like that to allow for the double wide skylight on the other side.
Exactly
Amazing what pictures will due. 
It looks like a 2x4 rafter . I could be wrong but if it is it should have been a 2x6 crowned up …that may have stopped the sagging.
They probably should have ladder framed between the doubled up trusses. Uh oh.
Well, there ya go. Would have been nice to see that pic along with the first ones…
Yeah Brad!
feck! live and learn, thanks all.