I watched this scene unfold but I'm still not sure exactly what is going on. My best guess is that this is how harvestmen fight.
The one on the left is using it's um...mouth arms, which actually have little crabby claws on them to grab the legs of the one on the right. The one on the right had slighly shorter mouth arms and spent the whole time flailing them around trying to grip something. They struggled like this for about 10 minutes before the one on the right gave up. Neither of them seemed to sustain any injuries.
There was no contact other than what you see here so if this is mating, I'm glad I'm not a harvestman.