Suppose there is a demon with Infernal Might 30. It possessed two individuals, instilling 15 of its own might into the first host and 15 of its remaining might into the second host. I don't have the book in my hand, but as I remember RoP:I says the "half-demon" in the first host has no information about the second host and the other "half-demon" inside after the possession has been done and the hosts has become separated. Is this correct so far?
The real question is that does not the demon sense at all if - for example - one host is destroyed and all the linked 15 might points are gone?
If the host is destroyed and the might points along with them then the demon will begin to regenerate the might points, which will give it information that the possession is ended but not how. It is far more likely that the 'half demon' in person A will flee before being destroyed however and convey its information to its other half. Also note that generally destroying a possessed person does not destroy the might of the demon possessing them.
Yes, I know that the possessing demon usually flee before it is killed, but it can happen that the mage is faster.
If all the might points that are used for the possession (or possessions) the demon enters the host (or hosts), so it has no spiritual form. In case of multiple possessions the demon can't oversee the hosts, it exists in the hosts separately. If one host is destroyed, the demon is not destroyed, it lives on in the other host and - as you said - probably starts to regenerate the missing might points (and recreating its spiritual form) but does not know what happened. If the half demon in host A is able to flee before being destroyed, how does it know where are the other host and its other half?
That is a YSMV question- essentially whether it can sense the direction to the rest of it's might, or whether it simply starts regenerating at that might if destroyed (since where else would it regenerate?) Otherwise it knows who it put its might in and can travel very quickly to look for them or to track down what information it might be able to overhear or otherwise gather.
Yes, in a classic saga probably that would be the case, but now I run an intro adventure and I try to concentrate a bunch of interesting elements into 4-5 game sessions in order to display to my players the uniqueness of ArM. For this reason the demon will be revealed by helper NPC-s to make things more interesting and challenging.
Just because the demon is revealed does not mean everything about the demon is revealed, and even with an astronomically high ability in infernal lore some things would still not be known since deception is the essence of the infernal.
Watch out for possessing demons leaving one host and possessing another host in the room... One reason for ensuring that any exorcism or exorcising magic is performed only by those with a degree of protection versus possession, whether via relic, Divine Resistance, Parma Magica, etc...