You choose which player gets which life total when the spell resolves.

You can’t split up a life total when you redistribute it. For example, suppose that in a two-player game your life total is 5 and your opponent’s life total is 15 when Reverse the Sands starts to resolve. You can choose to (a) leave the life totals as they are or (b) make your life total 15 and your opponent’s 5. You can’t choose to make your life total 20 and your opponent’s 0.