Yeah, I don't understand why sharing isn't allowed here, but... Eh, it's totally pointless, in my opinion.
My friend Nixie got her account banned. Someone reported her because her sister and sometime's this kid Nick would go on her account and talk to her Mibba friends.
I'm not sure if what you two are doing counts as sharing, but just a heads up, that it might happen.
Or, if you want to let people know she writes a lot of the stories or chapters or whatever, at the end of it when you can leave a note, you can credit it to her? Or in the description, even.
Personally I don't know why they freak when you share accounts. I just remember when I was registering, it said something about if you're sharing, fake, or have multiple accounts, you'll be deleted.