Did Jeremiah know that while he was prophesying to the Jews in Jerusalem, Ezekiel was prophesying to the Jews in Babylon?
It would appear that he did not.
From chapter twenty-nine (and from Ezekiel) we know that the captives in Babylon believed their kinsmen would come rescue them. The captives believed they were being punished but those left in Jerusalem were being blessed and these lies are being promoted by false prophets in Babylon. Jeremiah heard of the situation and wrote the letter contained in chapter twenty-nine to the captives. Help was not coming soon. They needed to get on with their lives.
With regard to the false prophets, a persistent question remains to which we have repeatedly returned: how was Israel supposed to tell a false prophet from a true prophet? Given the choice between Jeremiah and Ahab and Zedekiah, how were they supposed to know who was telling the truth?
The short answer is if a prophet foretold something that didn’t happen, he was a false prophet. But since prophecies often occur over the long term, how can you know in the short term who is telling the truth and who is lying?
The real answer is much longer. First, it has to do with the long-term reliability of the messenger. Jeremiah had a reputation as a spokesman for God. The other guys were newcomers. But more importantly, Israel was sent into captivity because of her sins. The sins had not decreased. There was no basis for believing their fortunes were going to turn around.
We are, each and all, responsible for knowing the will of the Lord, and knowing when our lives are not measuring up. To tell ourselves (or others) that better times are ahead when our relationship with God because of persistent sin is unchanged is deception. It doesn’t happen.