I hope that ARKit can be a lesson in what works well and what doesn't, and spur the open source community to create something like it, but cross platform. As more phones get dual cameras, what we can do with software like this will only get better.
ARKit doesn't seem to require dual cameras because it runs on iPads too. I do wonder if it uses the depth sensing second camera in the iPhone 7 Plus, or if that data isn't fast/precise enough to be useful.
Apple added a new depth api that gives real time information from the dual cameras. I’d be surprised if ARKit doesn’t use it to enhance the experience on the 7 Plus.
That said, it isn’t required. I have run the demo on my own iPad...the tracking is seriously impressive even with just the one camera.
Look at OpenCV for example.