The group I belong to quilt large quilts in sections.
Double say 2 sections leave joining the row of stitching all the way down centre.
Sandwich one half and quilt as you wish.
Repeat sandwich and quilting on the other half.
Tip: leave about one inch free of quilting down the centres of both halves.
No join the top of. Quilt as you would have done.
Place on a flat surface and level off batting/wadding and hand join.
Now put the backing over each other and hand stitch .
Then just finish off quilting lines( sorry this is bulky but one row only)
Larger quilts could go incorrect sections or do the sashing joining. Technique .
Hope it his is useful