Þjófafoss (or Thjofafoss without Icelandic letters; not to be confused with the falls by Mt. Burfell) is just downstream from Hundafoss. This rather obscure waterfall is usually ignored or unknown to tourists as it's not visible from the trail to Svartifoss, but it can be seen from the other side of the gorge from one of the hairpin turns on the single-lane road leading to the car park closer to Svartifoss than the visitor center (i.e. what I'm calling the cheater's path). If you look closely at the photo, you can see part of Hundafoss in the background.