I'd also wager a guess that most South African farmers are not white (they own 70% of farm land mass, but with no real evidence I'd imagine the farms are on average much larger than what was left for the locals), which would push that 2x stat much higher.
Edit: On the other hand, I do think it would be interesting to compare the rate to farms of similar sizes and different races, should that data exist somewhere. With murder rates 2-~20x above average I wouldn't be totally surprised if the common factor were just extra wealth or something.