This stitch is used in Cutwork Embroidery when you want a thick bar to stabilize a cut-away space. Normally, you would use a Buttonhole Bar or a Branched Buttonhole Bar.

After marking the pattern (shown here as blue markings), and determining what areas will be cut away, baste along the line you want to use for the Double Buttonhole Bar (step 1). Remember, it's all right to double back on your basting, if you don't want to cut the thread. Next, cover the basting thread with the Buttonhole Stitch, spacing the stitches two thread-widths apart instead of one (step 2). Make a second pass with the Buttonhole Stitch to fill in the gaps, with the purl of the stitch on the other side of the bar (step 3). Cut away the fabric close to the stitching, as with regular Cutwork.