Spoiler for Hidden:
Except:
Abby's storyline is important to put Ellie's story into context. Not just the motivations of the villian, but the consequences of Ellie's actions.
It also is important to give Ellie's story a motivation.
If you only played Ellie's story, the similarities and the consequences wouldn't quite hit.
If you only played Abby's story, it might work better, but it still has problems. And no one would want to play it for sure.
This is ultimately Ellie's story, but Abby's story is what makes her story possible.
I don't think the game would benefit getting split up, but maybe shortened still. Make Abby's day 1 shorter or something, or fit the story into two days.
Abby's latter story is so awesome, but the game's overall pacing hurts from the length.
Although I bet the pacing is better the second time around because you aren't expecting a 40 minute story anymore.