Feed with a 1/2 and 1/2 mixture of the two flours. For example, take 1 tablespoon of your bubbling brown rice starter at the top of its rise and mix it with 2 tablespoons of brown rice flour and 2 tablespoons of buckwheat flour plus 1/4 cup of water. Mix well to aerate (get oxygen into the starter). Once the mixture is bubbling and happy, feed it again in exactly the same way. Do two or three feedings just like this. Be sure that it is bubbling within 6-8 hours of being fed. You now have a half buckwheat, half brown rice starter and you can maintain it like this if you like. I prefer my starter to be half buckwheat, half brown rice because they complement each other well. If you want are grain-free or just want only buckwheat in your starter, then you need to keep training.
To continue training, remove 1 tablespoon of your new 1/2 and 1/2 starter and mix it with 3 tablespoons of buckwheat flour plus one tablespoon of rice flour and 1/4 cup of water for a few feedings.
Once it is bubbling on schedule (after 6-8 hours of being fed), you can now feed with just buckwheat.
If your starter doesn't rise as well with just buckwheat, go back to step 2 for one or two feedings. Or, you can always add a bit of brown rice flour to your buckwheat starter if you don't have to be completely grain-free.
To get your starter ready to bake with, you will want to increase the amount that you are feeding it. (Assuming that most recipes call for a cup or more of bubbling starter. In order to do this, you just keep the same ratios, but increase the amounts. For example, take 1/2 cup of starter and feed with 3/4 cup water, 1 cup of buckwheat flour (or one cup of any combination of buckwheat and brown rice flour. It shouldn't take any longer to rise (6-8 hours).