Many people say you shouldn't join the seam in the middle as this is the area that get the most wear.
I can't remember where I picked this tip up (probably somewhere on here, lol) but if I need to join 45" wide fabric for backing, I firstly unfold the pieces to be joined, then iron out the centre seam.
I then place the two pieces right sides together, selvage to selvage and sew seams, forming a 'tube'. It's up to you whether you sew 1/4" or 1/2" seams or trim the selvage. Press seams open or to the side, as you prefer.
Then, with sharp scissors cut the 'tube' up the centre - you'll probably still be able to see the original shop fold to guide you.
The joining seams will now be even on each side and you will have a large section in the middle.