Dual zippers (also referred to as two-way zippers) have two sliders:

One that unzips from the top

One that's oriented upside down and unzips from the bottom

Importantly, these zippers don't have a zipper box (reference image #3).

If an issue occurs with a dual zipper, first check that both sliders are present at the bottom of the zipper. The top slider can migrate up the zipper tape and the zipper won't fasten with only the bottom slider.