as far as lining up double sided quilts, I don't know how longarmers do it, but whatever I quilt
(This assumes your quilts are squared and flat....)
find center of each side on both tops and tie on a long piece of thread. I use thick crochet thread.
Long so you can see them once the batting is on
I spray baste the bottom one, center the batting (I tape my tops to the floor but there are lots of ways to do it)
then with the top folded in quarters I eyeball align the strings for that area, then unfold and super align the strings.
then I fold back 1/2 lightly spray baste, align and smooth that half. Then same on the other side.
Takes longer to type it than do it... sounds fussy but it isn't.