Commentary for Rosh Hashanah 4:4

אמר מר מת באדר ועמד אחר תחתיו באדר מונין שנה לזה ולזה פשיטא מהו דתימא שתא לבי תרי לא מנינן קמ"ל

The Master has here said, 'If he died in Adar and was succeeded by another, we can date the year by either'. Surely this is obvious? - You might think that we never date the same year by two kings;<span class="x" onmousemove="('comment',' But reckon the whole as belonging to the one who has died.');">⁴</span> hence we are told [that this can be done].'

Rashi on Rosh Hashanah