Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides.

In a large bowl, combine almond flour, granular sweetener, coconut flour, and salt. Mix well. Pour in the melted butter and vanilla extract. Stir until crumbly. Press about two-thirds of the mixture into the bottom of the prepared pan to form the crust.

In another bowl, combine chopped rhubarb, chopped strawberries, water, and allulose. Stir gently to coat the fruit.

Spread the fruit mixture evenly over the crust in the baking pan.

Crumble the remaining one-third of the almond flour mixture over the fruit topping.

Bake for 35-40 minutes, or until the topping is golden brown and the fruit filling is bubbly. Let the bars cool completely in the pan on a wire rack before lifting out and cutting into squares. This is crucial for them to set properly.