Before you enter the boss room make sure your coin is still active. Sometimes a previous miniboss will count against it. Also, coins don't work on SABER and another boss I can't think of. Coins will be consumed at the Tank before the Brothers battle. Concerning the Brothers, use one coin for each. So, going in make sure you have a coin burned, down the red brother. Then burn another coin and down the blue brother. Hope this helps.