Cut about 4 bananas into chunks and blend in a food processor until smooth. Put in a freezer-proof container and freeze until just frozen. Remove from the container and blend again to make a creamy frozen icecream-like consistency. You can repeat for another freeze-blend step if you like.
Divide the frozen/blended mixture in half, and add about a handful of frozen blueberries to one batch and a handful of frozen raspberries to the other
Blend each one separately to incorporate the berries, then spoon into the container again. You can swirl the 2 flavours together. Freeze again, or serve.