I think the first mistake people make when trying to fix a split zipper is attempting the repair with the clothing still on their body. While if you can free yourself you’ll feel a lot more in control of the situation. Sometimes you get lucky with a zipper that is open on both ends, like a jacket. But if your favorite jeans split, you have yourself quite a zipper dilemma. This trick from makeitorfixit.com makes perfect sense! You’ve got to make a fresh cut between the teeth of the zipper tape. Slide the zipper slider all the way down. Insert the slit part into the zipper slider. Zip up. then you have to repair the cut. Here’s how to do it at Make It Or Fix It:
How to Quickly Repair A Broken Zipper