Place the frozen banana coins and frozen mangoes into a food processor or Vitamix. Let them sit for about 5 minutes, just so they soften slightly.
Process on high speed, stopping occasionally to give the food processor a break. First the fruit will become chunks, then smaller chunks, and finally it will become very smooth and creamy.
Serve immediately.
Notes
It doesn't really work to store the finished nice cream in the freezer, it will freeze and become super hard. This is best enjoyed immediately.