In a blender, add frozen mango, Greek yogurt, coconut milk, and sweetener. Blend on high for about 30 seconds or until smooth. If smoothie is too thick, add a splash more coconut milk.
Pour between 2 glasses. Rinse blender.
Using same blender, add frozen strawberries, Greek yogurt, coconut milk, and sweetener. Blend on high for 30 seconds or until smooth. If smoothie is too thick, add a splash more coconut milk.
Pour strawberry mixture slowly over mango mixture already in glasses. Garnish with fresh strawberries or mangoes (if desired) and serve immediately.