Watching the anime, I've always wondered just how the hell did Yuu ever put up with Ayumu's possessiveness and codependency? How did she even stay calm through every single of Ayumu's outbursts? Then episode 12 hit and I realized that Yuu was actually enabling it.
YuuPomu isn't exactly healthy, Ayumu can't share and Yuu can't let go, but it is interesting that a relationship like this actually exists within Love Live.
It's not exactly that Ayumu can't share Yu. It's deeper than that. You see, Ayumu also has confidence issues and always seeked validation. And who else could she trust to give her that but Yu? So it's not that she doesn't want to take her to herself, but more like she's afraid that if Yu ever pursues this dream that Yu wouldn't have space for Ayumu anymore.
Plus, throughout the episode, we see that Ayumu was wrong. She realizes that other people do care for her besides Yu. So now she doesn't know what to do because she's seriously never cared about anyone but Yu before. And now even she is wondering that now that she's actually starting to love being a school idol without Yu that she won't have much time for Yu anymore.
Haha sorry for my long message! Just wanted to give my two cents on the whole drama ^^