Two palantíri could communicate with one another. If two were used this way, a third user trying to communicate with one of them would see a blank “screen”; but a user with the palantíri of
Osgiliath (and
maybe Amon Sûl) could eavesdrop on the conversation. (Extending that idea, a user with
the Master Stone in Avallónë could eavesdrop on any stone.) A user in Osgiliath could see what the user of another stone was viewing, whether or not he was in communication with someone else.
One can
infer from the text that a user in Amon Sûl could see what a user in Annúminas (later Fornost) was seeing, though it is not stated. If that is the case, then you can see immediately why the
Witch-king of Angmar was so eager to obtain the Amon Sûl stone. He was unquestionably under instructions to get at least one
palantír from
Arnor, the weaker Dúnedain kingdom. When he failed and they were lost,
Sauron immediately sent him, along with
all the rest of the
Nazgûl, to
Minas Ithil to get that stone. Besides its offensive and defensive uses, Sauron needed it to search for
the One Ring, just as
Saruman used the
Orthanc stone for the same reason; besides, Sauron’s possession of the Ithil stone pre-empted the
Stewards of Gondor from using their remaining palantíri for fear of encountering the
Dark Lord. (Depriving his enemies of such a useful tool was a little bonus for Sauron.)
There is a minor question: could the Osgiliath (or Amon Sûl) stone eavesdrop on the Elostirion (also called the “
Elendil” or “Emyn Beraid”) stone? I don’t know, but I don’t think so: I think that stone was for communication with the Master Stone alone.
The palantíri were indeed made by
Fëanor, who is elsewhere reported to have constructed devices that allowed a person to see
with the eyes of the eagles of Manwë, if I recall the citation correctly. (If not precise, it’s close.)
Gandalf told
Pippin he thought Fëanor made them. Whether Fëanor and his sons in their exile took them to
Middle-earth goes unsaid, but probably so, since the Master Stone was in Avallónë on Tol Eressëa, where the
Noldor returned from exile lived (still in exile, but a lessened exile) in the Second and Third
Ages, instead of Eldamar, where the Noldor who had not rebelled remained.