What are the rules? And how do they not end up hurting each other?
According to the article, the edges are dulled. Also they're going pretty slow in these videos and swinging at ranges where they can barely reach each other. It looks to me like a collaborative sort of sparring. If they were trying to hurt each other, or maybe even "win" at all, I assume it would be much faster and more brutal.
At time 1:56 in the the video, the older guy "lets rip" a little bit and speeds up. It gets a murmur and a grin from the crowd, giving the impression that they are aware of what it can be.
Definitely has a bit of a Capoeira "trust me bro this would work great in a real fight" vibe in all the videos. Would be interesting to see more full-speed sparring, and also see how it would evolve with protective gear and stand-in weapons that let them really go at each other.
You might like the messer sparring video someone linked further downthread. Similar flavor, without the sticks.