Place the white and brown tepary beans in a large bowl. Add enough water to cover by 4 inches and let soak overnight at room temperature. Drain the soaked beans, discarding the liquid, and transfer the beans to a large pot with a tight-fitting lid. Add enough cool water to cover the beans by about 4 inches. Bring to a boil over high heat, then lower the heat, cover and simmer gently, stirring occasionally, until the beans are tender, about 1 1/2 to 2 hours. Reserve 1 cup of the bean cooking liquid, then drain the beans. In a large, deep skillet, heat the oil over medium. Add the onion and sauté until translucent, about 3 minutes. Add the cooked beans and the reserved bean cooking liquid, the agave and the chile powder. Cook, stirring occasionally, until the liquid has reduced to a glaze, about 10 minutes. Season with salt to taste. Divide among bowls, sprinkle with additional chile powder and top with oregano.