I piece my backs regularly. Wish I had a bigger budget for wider fabric but I don't and I seem to be unable to make quilts that fit nicely into one width of fabric.
The next top I will quilt down has a pieced flannel backing. I sewed my seams at .5" and pressed open.
When you piece a back, you want to avoid having the seam on an area likely to be folded, so not down the middle. Typically for a larger back you use a whole width down the middle and then the other piece you cut down the middle so you have 1/2 WoF (width of fabric), full WoF, and then the other 1/2. Yes, two seams but it's better than one!
The quilt I finished last night I had a lot of yardage to use for the back. The quilt itself was maybe only 4" wider than the width of fabric... I went the horizontal route for the seam, there is only one about 1/3 up from the bottom. I used the remainder of that yardage to make the binding.