Take a grapefruit and slice off the tops and bottoms. Rest it cut-side down, and slice down in knife-width sections to remove the peel and pith all the way round. Repeat with the other grapefruit, then take one and cup it in the palm of you hand. Working over a bowl to catch the juice, carefully slice down the sides of each segment to the middle, following the membrane as closely as possible to maximise the segment size. A small serrated knife, or even a really sharp filleting knife is the best tool for the job.