Finely chop 1 mango and 1 cucumber and set aside.
Chop remaining mango and cucumber into big pieces.
Blend them with 1/4 cup water in a blender until smooth.
Transfer to a bowl and stir in finely chopped mango and cucumber, onion, lime juice, and 2 cups cold water.
Place bowl in a larger bowl of ice and cold water and stir until cool.
Just before serving, stir in coriander and 1ΒΌ tsp salt.