Ok, I know that I am not the only one who raised an eyebrow when Jessica and Niki were talking to each other in the woods, so here is my theory.
Jessica does not reside within Niki as a spirit, but as a complete symbiotic physical being within Niki.
Evidence:
When Niki/Jessica falls in the woods there is a brief glimpse of her reflection in the mud puddle. Then Niki looks up from the ground and sees Jessica standing on the other side of the puddle. She is looking up and not looking down into the puddle.
When the camera angle changes so that we see Jessica speaking, this background appears--

Niki continues the conversatin looking up until the end of the conversation. Then the background behind Niki is very different from the background behind Jessica--
Secondly, the tattoo

that Jessica bares does not appear on Niki

I can't make my tattoos disappear. I doubt that disappearing tattoos is a special power so that means that it exists on one body and not on the other. That would mean that there are two bodies there. Perhaps it is someowhat akin to the symbiot that webheads are familiar with. That bit of speculation is still up for grabs, but I think that they are two different entites that exist in one space.
Woo Hoo a new nutty theory by the captain of the looney theories team.