It all sounds sort of woo-ish, yet not entirely implausible since it doesn't seem to be supernatural or violate any laws of physics. It's analogous to multi-tasking in a computer. I've always thought that even our primary consciousness is an entirely virtual construct, essentially software operating within the hardware of the brain, so it's not too much of a stretch to imagine two "consciousness programs" running in the same brain. But I don't know nearly enough about the brain to say whether that's really possible, and I'm not sure that anyone does.
Trying to approach it empirically, to test if it happens without necessarily understanding how it happens, also seems not very practical. It's not a very testable idea. How would one distinguish between an actual separate sentience, and merely a delusion of such? And how would anyone other than the one experiencing it have any way to distinguish whether the subject was actually experiencing anything at all, real or not, vs. just consciously perpetrating a hoax.
I suppose that some objective measurements might shed at least a little light on it. Maybe using a PET scan to see if the presence of a tulpa produces any characteristic change in brain activity. If a tulpa causes more intense or more widespread brain activity, then it might also be observable in a simpler way, as an increase in brain temperature. Another possibility might be to measure ones response time and/or accuracy on some kind of mental test. If a tulpa time-shares the same brain hardware as the primary consciousness, as opposed to activating more hardware, then the presence of an active tulpa might manifest as slowed responses or diminished ability rather than increased activity.
Whether possible or not, it is sort of fascinating as a thought experiment. It raises many questions. Many of them potentially complicated and right at the root of what we are and what it means to be sentient.
Can a tulpa fall in love? And how do you deal with it if the tulpa develops an independent love interest?
Does a tulpa share all of the same sensory input with the primary consciousness, or does it exist in a mostly sensory-deprived state? What effect does that have on its mental well being?
Does a tulpa have any motor control at any time? Are you the arbiter of that, or could the tulpa have as much ability for that as you do?
Is a tulpa concerned about the physical well-being of it's host, since it dies if/when you do?
Can a tulpa become the dominant personality, taking control of the body and displacing the original consciousness?
Can a tulpa, once created, be consciously un-created? Would that be murder? Is a tulpa scared by this prospect? Could it do that to you?
Can a tulpa cease to exist for other reasons? Maybe just neglect, or diverting too much of your brain's resources to other tasks? Is this equivalent to death? Does the tulpa think about this? Fear it?
Can a tulpa be evil, even if the host is good, or vice-versa?
What do you do if your tulpa has become a homicidal maniac, or A bigot, or just an all-around idiot that you can't stand to be around it anymore? (imagine it as a Ray Comfort or Pat Robertson)
Could the divvying up of the brain's resources cause both you and the tulpa to have diminished mental abilities? Two idiots in a brain that used to support one intelligent person? I'm pretty sure that old canard about using only 10% of our brains is bunk.
It seems to me very likely that creating a tulpa is nothing more than imaginary friends taken to a new level. But if it turns out that it's more than that, that a tulpa really can become a distinct sentient entity, then it seems like an exceedingly dangerous thing to play with, for both the tulpa and yourself.