Two people grabbing the ledge at the same time?

[hotdogturtle]

In this video it clearly happens.

https://www.youtube.com/watch?v=cq2fkkQnVGI&t=15m4s

I know that tethers allow multiple simultaneous grabs, but I thought that all normal grabs all followed the ledge trumping mechanics. I'm surprised that I haven't heard anything about this at all, because it seems like it should be a bigger deal than it is. So does anybody know what's going on here?

 

[Kalierdarke]

Looks like they both request the game to trigger a ledge grab at the same time, and therefor the game doesn't know who to throw off, so just says "ah screw it, you're both there"