You mean so simple that a money loaded organisation like FIFA won't do it for goal line controversies. hehe. But yes, right step in the right direction. Looking at replays on TV, it looks like the frame rate would have to be much faster to improve the positive predictive value. I agree that 2 challenges seems too few, unless you mean 2 challenges per game. If you had more than 4 challenges (>2x2) over the whole match, would that be too disruptive? Personally, I think it would be good for the game. Commentators have a bit more time to do analysis and commentate.