Usually the 10" max means that you can have 10 inch spaces between your quilting lines, but no further apart or the quilt will bunch. Any smaller amount of spacing will work--anywhere on the quilt. The two inch diagonal spacing will work fine. In fact, that should work really nice, and no, you do not need do another line of stitching if you're going to do the diagonal lines. (Might want to pin or baste the edges down before you begin the diagonal to keep them getting off grain.)