"Oh."

"Well. I still think it would be cool to run some tests. You make a body, Heidi tries to get in, that sort of thing."

Mary's expression falls away as Heidi finishes collecting her thoughts. Their expression becomes blank, then intent.

"I'm Heidi. I suggested the idea, so perhaps I should tell you why I think it's plausible."

"I think I could try to meditate inside your conjured body or try to move it. I very much doubt I'll end up with another instance of myself, because I don't want to, and Mary doesn't want there to be one either.

"... I suppose that needs context. Tulpae are created when their host imagines another person vividly enough. We develop independent reactions and then independent goals and preferences. We can have bodies if a host practices visualization often enough, and likewise voices, and these grow to be our own over time as we become more independent. But these bodies and voices are still only perceived subjectively by our host. 

She goes silent for a second, raising her hands to her brow, her eyes closed.

"The more relevant point is that tulpae are not created instantly. Leaving aside the fact that we require interaction and direction in order to exist at all, our growth is guided by our hosts and our sentience is determined by whether our host wants it. I suspect something similar might be happening with your power. There might be a mental block stemming from revulsion at the idea of creating a person."

He blinks at that. "Hi, Heidi, nice to meet you," he hazards. "I don't think that was what was going on—I'm unusually good at dealing with my own brain, though not as good as my girlfriend—actually I should show her this place somehow—but I'm not against testing, testing's fun, I'm just saying it probably won't work." His copy disappears. "Do you have a way of showing me your preferred form?"

"Sure."

Heidi conjures a figure of a tall, bald black woman with a strong jaw, wearing a black dress and hoop earrings.

"How tall do you want to be?"

"As tall as Mary, please. I'm used to her height and center of gravity."

Mary is around 180 centimeters tall. 

And there she is.

That's exactly what you look like! Mary chimes. 

Meanwhile, Heidi is concentrating on inhabiting the body. Is she getting any sensory information back? Proprioception, the wind on her skin, the feel of the clothing? How does it compare to fronting their body?

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?"