What you can do is remove the existing zipper pull and install a double-sided pull of the same size. Outdoor Wilderness Fabrics (owfinc.com)has a good selection of pulls. If you are not sure about the size, just order one of each. They are cheap. This is not as practical if it is a separating zipper, but it can be done. Lightweight tents and tarps usually use #3 (smallest) or #5 (medium).
If it is a non-separting zipper, remove the stop at one end of the zipper (I prefer to work at whichever end results in the zipper being closed), pull the existing pull off, get the teeth clear so you can access them and line them up (sometimes tapes are folded over and get in the way, if so, unstitch the folded part), sear the edges of the tape so it does not fray, then work the new pull onto the teeth. Make a new stop by stitching over and around the teeth with button thread.
For a separating zipper, you may have to buy a matching separating zipper of the same gauge but with a double pull. Sometimes you can buy them separately, but not usually. On the new zipper, remove the stop at the top of the tape that the pull stays on, and remove the pull. On the currently installed zipper, cut the stop from the top of the tape that holds the pull, put the new pull on and make a new stop with several stitches of button thread.