Rough Openings open now / header gap / should it be insulated??

My windows have a 2 piece (multi piece?) header of some sort. What I see from the interiror with the window closed and the DRYWALL RETURNS REMOVED is a gap that runs straight up the wall above the window space. The jambs and sill of the R.O. are 2 x 4 but this header arrangement seems to be something like a two 2 x 10 (?) boards with a 1 inch (or so) air gap between them. Is this the carpenter trying to emulate a full thickness header using thinner boards? I really know nothing about framing so I am at a loss... should I insulate in these big gaps? Any guidance welcome as the clock is ticking on this one... The reason that the drywalls have been removed from all the jambs is because it was my job to prepare the interior for new windows - contractor will do the exterior and actual window replacement in a few days. - THANKS!

 

The 2x4 wall framing is 3 1/2 inches thick. 2 2x10's are only 3 inches thick (1 1/2 inches each). Hence a 1/2 inch gap between the two header members. Some carpenters sandwich a piece of 1/2 inch OSB between the header members as a spacer, but it is not absolutely necessary.

 

Yes, you could insulate the space in the middle of the header. You could also have 2x6 walls where different carpenters use varying methods of framing up headers.