Here is a link to Craftsy; scroll down to see what they recommend for piecing thread.
https://www.craftsy.com/quilting/art...lting-threads/
I believe the reason for using a single thread is because using two threads means the threads are rubbing against each other, actually making the seam less strong. Whether this is myth or experience, I don't know. A single thread is enough anyway, provided the quilting is reasonably close together. Quilting reduces stress on the piecing threads (and the fabrics in the quilt). The instances where I have seen close-ups of pieced seams that have come apart all appear to be cases where the piecing stitches were large. Piecing stitches should ideally be about 1/8" apart, and under 1/4". Smaller stitches are stronger.
I love Aurifil thread but I think, for me, 28 wt would be too thick -- harder to thread through the eye of a needle, and harder to pull through fabric. I love Aurifil 50/2 for machine piecing, but I wouldn't use it for hand piecing because it is only 2 ply. I really think a 3 ply thread is preferable for strength.
Edit: I see that Aurifil 50/2 is recommended by others for hand piecing. Scroll down to hand piecing here:
https://www.thesprucecrafts.com/choo...ilting-2821961
This hand piecer also prefers Aurifil 50/2:
https://redthreadstudio.com/blogs/gu...ecing-tutorial