Ah, I see the problem.

Rather than stepping in and doling out constant punishment and corrections, recall Bianca. The recall takes care of both major issues: it stops Bianca from bullying Koie and it redirects her attention back to you (it gives her a positive, alternative behavior).

So, every time you see Bianca bullying Koie, immediately recall her. Reward her. Send her back out to do something else or redirect her with a toy, command, whatever. Initially, you'll be doing a lot of recalls, but that's perfectly normal and it's good. The more you recall and the more you reward her for a good recall, the stronger this behavior becomes and the less she will bully Koie. It becomes a transfer of value. The value of coming back to you becomes higher than the value of bullying Koie. As you can probably tell, you'll need a very strong recall to begin with, so start reinforcing your "Come's" now. Reward often and make it a game.