Shoulders seams:
Thread the tapestry needle with the first tail of top and with back stitch seam (working on wrong side) join both layers, working through all four loops of first cluster shell (for bigger sizes you can join first two clusters, or one and a half, it is up to you, only take care that the neck part is wide enough).
Thread the tapestry needle with second tail and join both layers of second shoulder in the same way.