Having fabric big enough for backs is a constant problem for me. I do prefer to use extra wide but I don't always have it or the budget and I have a lot of stash... I rarely buy anything more than 3 yard pieces so one thing I've learned a lot about backs is piecing them. Most backs are 6 yards minimum to 9 yards plus.
Ok, as already mentioned 88 inches is the maximum two pieces of fabric sewn together is going to give you. Mostly, I count on fabric as only being 40" usable and I use the mantra it is better to be half a yard too much than 2 inches too short.
When piecing backs, you want to avoid seams on the fold line, that is directly in the middle of the quilt. The "standard" way is to have one long piece in the middle and cut the other piece in half the long way, with those narrow halves on either side of the full middle. But sometimes your fabric fits best in the middle and so you say fine, be like that and don't worry about it.
For your project, I don't see much in the way of getting out of buying 9 yards. You won't use it all. You can do it for less.
I decided that if I was going to piece backs anyway, I might as well start adding in some extra fabrics. I use leftovers from the top, related theme fabrics too big of scale, or other things.