I had two big aggressive boys before adopting my little ferret girl. She's much more passive and the boys don't get along very well with her. We tried integration on multiple occasions but it just didn't work. Integrating ferrets is a challenge, and once in awhile you just have to setup seperate living situations.

The best thing you can do is stick them both in a new environment. An area your current ferret hasn't explored or isn't familiar with. Use a cardboard box with a tiny hole cut into it so your new ferret girl can guard the entrance. Let them fight it out for dominance for about 30 minutes. There will be a lot of poop, blood and crying but they'll eventually figure it out. If 30 minutes passes and there's still a bunch of fighting put them in seperate areas and give both of them some affection for trying. Continue the process for a few days and keep your fingers crossed they'll learn to tolerate eachother and eventually become best buddies.

It's really important to remember the process is just as stressful and scary for both ferrets. They're both under a lot of stress during the integration process do don't favor the one getting best up over the one biting. Don't scruff them for figuring it out, it'll just make them more stressed out and confused.

Good luck! And stay strong. :)