The double box is really two boxes. In my pattern ones I have one colored box and one white box, although to save ink it would make sense to have two white boxes.
The boxes need to be sized so that you have a nice even reveal around the edges. I like a 1/4" edge, but mostly because the math is easy. If you have a 1/4" edge on all sides the bigger box needs to be a half inch bigger both in height and width.
The first box you draw will be the bigger box, in one of my examples I made the box 6.25"x1.75". Next draw the second box, 5.75"x1.25".
Select both boxes, right click and find alignment and pick center. do that again and pick the other center. Then right click and click group. Once they are grouped you should be able to move or re-size the boxes to what ever spot you want them to be.
Does that help?
MrsN