Combine brown and black rice in medium pot. Pour plenty of boiling water (any amount) over rice directly in cooking pot, cover, and let soak for 20 minutes.
Drain off soaking water, add measured water for cooking, replace lid, and bring to a boil.
Simmer over low heat for 25 minutes. Remove from heat and, without removing lid, let rest for 10 minutes before using.
Let cool before packing in bento (unless using in onigirazu) and/or storage container. Refrigerate for up to 4 days.