Sandwich the back bodice between the two yokes, sew and press. Top-stitch the seam if required.
Roll the whole back body up with the right side on the inside then pin to keep it out of the way.
Open out the yokes so that the internal yoke is facing down. With right sides facing, pin the shoulders of the external yoke to the shoulders of the front/fronts and stitch. Roll fronts up with the wrong side on the inside and pin to keep out of the way.
Now bring the internal yoke yp to the external yoke with right sides facing and pin at the shoulder seams (the rolled pieces should now be enclosed inside the 2 yokes). Sew the shoulder seams.
Gently pull the body of the garment through the neckline and turn out to the right side. Remove pins, press and topstitch the seam if required. Stay-stitch the two yokes together around the neckline to prevent stretching.