She has absolutely no sensory feedback from the new body.

Can she send information, like movement and expression changes, even if she's not receiving? 

Nnnope.

Hmm. Heidi will try to think of other tests.

"Why do you think it probably wouldn't work?"

"Well, why would it? It's not like you can transfer your consciousness to a regular brain, is it?"

"Well, I'm not magic and I don't run on superpowers."

She frowns. 

"Maybe if Mary visualizes at the same time?"

"Wait, not magic? Then... how do you expect it to work? And how are you not magic?"

I'm going to take over for this one?

Sure. 

"Maybe we didn't explain properly. Creating a tulpa -- creating another mind that also inhabits you body -- is something that anyone can do. It doesn't require magic. All it requires is the ability to act and believe as if the other person exists, and interact with them for long enough that their responses become automatic. Seeing Heidi and hearing her voice, which I can do, is - it's like a consensual hallucination, or -- I think the term is operant conditioning.  I imagine seeing it, I want to see it, I see it, it stops requiring conscious effort after a while, at which point Heidi can take control. Does that make se--"

She stops speaking suddenly, then smiles apologetically. "I don't think I introduced myself. My name is Mary Rivers."

"Sadde Woods," he returns in kind, looking somewhat bewildered. "So... Um. I'm actually not sure how to react to that, I mean, I think it'd be offensive for me to doubt its possibility."

"Lots of people react with doubt, we can take it. Although I would prefer if you act as if Heidi exists around me, even if you don't believe it yourself."

"I mean, uh... I'd like to believe things that are true, and this is an interdimensional bar so I'm pretty open-minded about what things could be true, it isn't that hard to believe there's someone else in your head. I just really don't know if I could do it."

"And if you don't think you can do it, then you won't. On the other hand, if you do think it's possible, it is."

"...um. There is presumably a fact of the matter as to whether a brain of a human from my world can house more than one person simultaneously, or create a new one, like that, that is independent of my belief, I'd think."

"Why do you think it's independent of your belief?"

"Because a brain is a machine that's running an actual software and has an actual capacity. I, uh, maybe in your universe there are gods? Other than Scion if that I'm pretty sure mine is godless and soulless so there's a finite capacity to a person's brain and whether it fits two people in it and can run them should be a deterministic question with a deterministic answer that depends on its physical characteristics and nothing else."

"In our world we have a mental illness called dissociative identity disorder, which looks similar to having a tulpa except that it's involuntary. And if another personality fronts, the host blacks out -- that doesn't happen with tulpae. Do you have that in your world?"

"Yeah, we do have that, though I think the evidence on whether it really exists is mixed. But I know that people can be full people after a hemispherectomy so it sounds plausible that two people could inhabit a brain, it's just that creating a whole new person sounds... well, difficult, at best."

"Well. It depends on how difficult you find modelling other minds -- it took a while for us but it wasn't that hard.

I suppose I'm a bit biased, having done it."

He grins. "I suppose so, too. But, knowing that it's a perfectly natural and nonmagical process, why would you expect it to work on an," he gestures, "empty construct?"

"I suppose I don't know that it would work -- my belief in Heidi's existence doesn't necessarily interact with your power."

Well...

"Did you expect it to?"

"I didn't know how your power worked! You said it was unlikely, but I thought it would be worth a shot, and you said you hadn't tried it before."

"Yeah, fair enough, I suppose." He looks around at the otherwise empty bar then says, "So I have a secret and it'd be very dangerous to me if it escaped to my world but you're not from there anyway, and also I'm pretty sure now it's the reason the door picked us."