Whisk rice flour, oat flour, teff flour, buckwheat flour, baking soda, and cream of tartar together in a bowl.
Puree beets in a blender or food processor until smooth; transfer to a bowl.
Stir pineapple with juice, orange juice concentrate, raisins, walnuts, agave nectar, flax seeds, orange zest, vanilla extract, xanthan gum, and liquid stevia into pureed beets. Add pineapple mixture to flour mixture and stir until batter is just-combined. Spoon batter evenly into two 9-inch round nonstick cake pans.
Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, 35 to 40 minutes. Cool in the pans for 20 to 30 minutes before removing to a wire rack to cool completely.