Cutting roof rafters in a gable ended dormer with a chimney breast

I've got a problem getting access into a part of the roof to work on the chimney and gain access to insulate the roof properly. It's a bungalow with dormers built into the roof where the chimneys are. Basically the jack rafters which are 4x2 are too close together to allow access through so I need to cut 1 or 2 to create an access hatch. Can this be done by doubling up the rafters and then creating a box frame around the new opening in 4x2 or 2 lots of 4x2 side by side and then tying in the cut ends of the jack rafters to the new frame?
I'm a little concerned of the load on these rafters as they go down into the chimney breast. I'll upload some pictures of inside tomorrow as it's a little hard to explain.
This is the only picture of the outside I have to hand.

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The access hole will be directly behind the chimney and through the main roof rafters. Allowing access into the bit of roof where the chimney passes from the room below and then out through the tiled roof.