We - The form used here occurs nowhere else in the Old Testament, but is the regular form of the pronoun in the Talmud. It is one out of many instances of Jeremiah using the popular instead of the literary language of his times.
Related Commentaries of Jeremiah 42:6
Jeremiah 42:6
6 Whether it be good, or whether it be evil, we will obey the voice of the LORD our God, to whom we send thee; that it may be well with us, when we obey the voice of the LORD our God.