The answer depends on a couple of things, such as how wide the fabric is that you choose, and whether there is a pattern on the fabric or a nap or pile that has to match in the same direction on each piece of fabric that you cut out of it. If there is a pattern or nap you usually have to cut the fabric in the same direction so that it looks right in use.
You can buy some fabric that is 60" wide of more, but not much comes that wide.
If the fabric you choose is at least 60", you would need to buy about 56" in length.
If your fabric is at least 56" wide, you would need to buy about 60" in length.
If your fabric is 28" - 52" wide you would need to buy 95" in length if you want the pattern or nap to run the same on each piece, or 88" if the smaller piece can but cut sideways, if the pattern or nap doesn't matter.
The easiest thing to do is, after you choose your fabric, sketch an open-ended rectangle on a piece of paper and mark it with the width of the fabric you choose. Then draw your two fabric piece measurements on it to fit on the rectangle's width, and see how you can place the two pieces in the most cost effective way for cutting. Always buy a little extra fabric anyway, to allow for selvedge and cutting.