This is how I seam two edges of English Rib together to make a flat and virtually invisible seam. To be successful, the ribbing must be configured to knit with the outermost left hand needle on the main bed and the outermost right hand needle on the ribber bed.
Work from the wrong side of the fabric and sew the seam by catching the outermost stitches of each edge. On one side, you will go thru the layers and pick up the ladder stitch. Then on the other side, go down thru the knot stitch of the outer edge. Then go to the first side and pick up the ladder stitch again. Continue in this manner along the seam by working back and forth between the sides. Remember not to pull the seam stitches too tightly so the seam is as stretchy as the garment fabric.