User blog comment:BlackWizardZeref/Fairy Tail Chapter 394: Juvia vs Keith Review/@comment-24039250-20140725090304/@comment-25244540-20140725090520

OK, one of the biggest reason in there I feel it doesn't work is even if END possessed Natsu he'd likely need to use Natsu's body.

So even if the mind is if END, the body is of Natsu's which to me means they'd both be there. And for all we know Zeref could have told END he can't possess Natsu for some unknown reason. It could relate to how during Sirius Island he wanted Natsu to kill him.