Spray slow cooker with vegetable spray. Add rice, quinoa, olive oil, salt and chili powder.
In a saucepan, bring the vegetable broth to a simmer. (The point just before boiling.) Whisk in tomato paste and garlic and pour over quinoa and rice.
Cover and cook on low for 2 hours. Remove lid and place a clean towel over the slow cooker. Replace lid and allow to sit for 5 to 10 minutes.
Please note that the stated cooking time on the Uncle Ben's brown rice I used was 20 minutes. For brown rice that requires longer cooking you will need to start the rice first and add slightly more liquid.