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.
Pretty much this really, if we use the analogy of a computer, a tulpa would be a separate hard disk drive you put into the computer.
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.
Generally you can tell if it is sentient if it starts acting self aware, has its own mindset, etc.