You need to order a seperating zipper.
If it's too long, you can cut it off at the top, to size. To put in a zipper pull stop, take a manual needle and about 20 loops. If that's not enough to keep zipper pull from going past it, do some more. I just tie it off with a square knot.
They also have double seperating zippers, but you probably don't want that. Then you can also zip it up from the bottom. It's good on rain jacket for more ventilation. Harder to zip up and easier to break.