So this is how one scholar imagines Jeremiah's prophecy in Jer. 45 was done (HSM is what I used as a stand in for the divine name... as in HaSheM, "The Name"):
Baruch makes a lament to Jeremiah. Jeremiah takes Baruch’s lament to HSM. HSM responds to Jeremiah with two oracles for Baruch.
At a later date Jeremiah tells Baruch that HSM has words for him. Jeremiah repeats Baruch’s lament. Then Jeremiah tells Baruch what HSM said in response to the lament. Then Jeremiah gives Baruch the two oracles from HSM. Baruch writes them down.