Similarities between 200 free and 200 fly long course?

Thought this would make for an interesting discussion. A fair amount of good 200 freestyle swimmers are also good at the 200 fly. Swimmers such as Michael Phelps, Chad Le Clos, James Guy, Duncan Scott, Kyle Chalmers. So what are the similarities? Is it the energy zones being used? The groups of muscles being used in a specific way? What is it about these strokes that often seem to pair them well? Following on from that, is there an optimal way to train for these two events together?