Pretend this rectangle is 44" wide. Not wide enough for your backing, right?
Notice carefully what happens when you cut diagonally, slide the piece on the bottom left UP, and the piece on the upper right DOWN. The width of the rectangle (your fabric) gets WIDER.
It also gets shorter, which you will have to take into account. But, this is a good, reliable way of making 44" fabric wide enough to use for a backing.