When I made a shadow quilt for my daughter, I did a SITD on both rectangles. They were close enough together that I wasn't worried about it and it gave a nice look to the back. Hers had an off white background with dark grey shadows and a variety of grey/mauve/burgundy prints for the boxes. I used a dark grey thread and it all blended nicely. Using that idea, I would do that with this one using a black thread. If you start at a junction of the print and black, you can do one continous line around both rectangles. No matter what you decide, I'm sure any walking dead fan would absolutely love this quilt!