WARNING - This is a fairly graphic clip showing a nasty accident between two bikers. Please don't view if you think you'll be offended by such footage.
Accident
Three mistakes I can see that led to the accident occuring:
1. Big mistake - First biker didn't make any real indication to his mate that he was going to turn.
2. Fair enough he did a long 'lifesaver' check over the shoulder a short time before turning, but maybe should have done a second quick lifesaver just as he entered the turn.
3. Possibly not a mistake but the second biker on a better day might have sensed the first biker's lifesaver as an indication that he was going to do a manoeuvre and held back.
I do hope they came out of it all OK, it looked pretty horendous...