Slow Cooker Instructions: Add an additional 2 cups water/broth and cook on high for 4 hours. Check at 2 hours and add more water/broth if necessary. It is better to have too much liquid than not enough, so if you are unable to check part way through, just start with more, making sure liquid is at least 1 inch above beans & rice. Slow cookers vary.
You can save a pan and saute the onions, garlic and spices in the instant pot before cooking the beans and rice. You can either leave them in to cook with the beans and rice, or remove them and reserve for later. I am often getting it started too late in the day and need to get the beans and rice cooking, so being able to chop onion, etc. after that reduces my time until dinner. I also find that cooking the beans and rice in the seasonings mutes their flavor a little, but you can also add additional seasoning if desired.
To soak beans – either place beans in water overnight, or bring beans and water to a boil and boil for 2 minutes, then allow to sit for at least an hour. With either soak method, drain and rinse well prior to cooking.
I generally soak my beans and rice separately, but I expect it would work to do an overnight soak with both together. Did you try it? Let me know how it worked in the comments!