In the Mishnah Berurah (271:3), it says that the word זכור from "זכור את יום השבת לקדשו" of the first Aseres Hadibros, and the word שמור from "שמור את יום השבת לקדשו" of the second Aseres Hadibros were said בדבור אחד ("simultaneously" for lack of a better word). Similarly, we say the same in the Lecha Dodi song in Kabalas Shabbos.
But how can this be? At least according to Rabbi Tanchum bar Chanilai who was told in the name of Shmuel bar Nachum (Bava Kamma 55a), it appears that the first dibros written in the Torah (in Shemos) were written on the first tablets and the second (in Devarim) were written on the second tablets.
So how were they mentioned together?