Why should these two,
Turgon and
Finrod, not tell one another what they dreamed? They each believed that he alone had received a message from
Ulmo, true, and it is best not to bespeak one’s secrets to many others, true; but each led a great people into hiding, and those folk certainly knew the secret.
Nargothrond remained secret because of the Guarded Plain, Talath Dirnen, but fell because of
Turin’s bridge across the Narog, which gave access to the
city to
Glaurung. Its position was not secret to
Morgoth when it fell, of course, but it could have been defended longer without the bridge. Gelmir and Arminas were sent by
Círdan to warn
Orodreth to tear down the bridge, but Turin gainsaid them in his pride, to his own ruin and that of his true love,
Finduilas. But at this point in the story, I think we can safely assume that the location of Nargothrond had been determined by the Enemy.
Gondolin was secret until
Maglor revealed its location out of lust for Idril as much as out of fear of Morgoth. Turgon wisely did not permit those who entered to leave it, though he made an exception for Aredhel his sister, to grievous end.
But I don’t believe that gets us any closer to why these two did not speak to one another about their dreams. Perhaps as Bucky suggests, they were each the leader of different groups or factions of the
Noldor. From “Quendi and Eldar” in
The War of the Jewels:
Indeed the Teleri on their side asserted that most of the
Ñoldor in
Aman itself were in heart Avari, and returned to
Middle-earth when they discovered their mistake; they needed room to quarrel in.