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Single lift in terms of one player lifting another, double with two players lifting a third i.e line out. The biggest difference is if the player in the air is touched , bumped , reaches for the ball or over corrects the lifter in a single lift can easily lose control and act as a pivot point for the player in the air to rotate and land head first.
I think it is a grey area with regard to whether there can be a difference in interpretation of the challenge as fair or dangerous because of the numerous situations. For example Folau’s style of jumping is to jump with a lot of forward momentum, if he and the Irish player in the air got their hands on the ball at the same time most likely Folau’s momentum would cause the Irish player to rotate backwards over his single lifter and land ugly. Is that a dangerous challenge by Folau? If there was no lifter there is no pivot point and the player would most likely land on their bum or back.
Don’t know the answer myself.