Whenever someone has multiple people/minds/consciousness in a single body, it's always going to be hard to concentrate on a single thing.
For them, it's like having ADHD, except in this case, it's ADHD on steroids, (and if they're going to bring any more Reyn into this fusion, it'll make it worse, however that doesn't mean they'll lose the fight or it's impossible for them to concentrate, it's only just harder).