You could use a narrow blind hem stitch on the front so that the straight part of the blind stitch was on the underneath fabric, and the zigzag part of the stitch was on the top which you have matched to the bottom. Then press it, and carefully turn it over, open and stitch along the fold with a regular straight stitch. The blind stitch would be just about invisible on the top, but if it mattered, you could snip them.