Step 13: Frame for the Windows, cont.
I finished the double windows by backing the frame with more 1x4's. I just butted them together.
The first picture shows the first two boards (horizontal, in the background and foreground. The board perpendicular is shown in the proper position in picture number (2). I put two 1" wood screws in each corner, driven straight through into the undelying frame. You'll want to countersink them.