Hi Steve! You certainly can feed your starter with two kinds of flour if you like to, consistency is the key so as long as this is your normal process it shouldn't impede the rise in the final loaf. Whole grain flours will cause the starter to ripen faster than may be expected though, we wonder if maybe the starter is ripening before you realize? We'd suggest using your starter sooner after a feeding to see if this helps you get a better rise. Happy baking! 

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