That's exactly what it's for, 2 rods on one downrigger. You'll attach your first rod like normal, just behind the cannonball. The next rod will be attached by the shuttlehawk. To make it work you attach a stopper (small plastic clip) on your cable a few feet above your cannonball. It should ride through your downrigger when you pull up your cannonball. Then you'll slip the shuttlehawk onto the cable and clip in your second line. The shuttlehawk works as a downplane as soon as it hits the water. It will slide all the way down to the stopper, riding just above your primary line. When a fish hits, you pop it out of the shuttlehawk and it should plane back up to the surface.
I typically run my most productive rig on the shuttlehawk, because it's so easy to reset vs. pulling your cannonball back up. My only issue with the shuttlehawks were the original clips they provide. I replaced them with some Offshore brand clips and I've been happy ever since. Hope it helps, Ryno