Heat the oil in a heavy medium skillet over medium heat. Add the onion and sauté until translucent, about 5 minutes. Add the chile powder and cumin and stir until aromatic, about 15 seconds. Add both beans with their liquid. Bring to a brisk simmer. Reduce the heat and simmer gently until thickened to a coarse puree, frequently stirring and mashing some of the beans against the side of the pan, about 10 minutes. Stir in the marjoram, and then season to taste with salt and pepper. Set aside while cooking the vegetables. If beans thicken too much, stir in a little water.