How were זכור and שמור said together?

Question

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?

Answer 1

On a less miraculous note, the ibn Ezra (Shemos 20:1)rejects a literal reading of the chazal that shamor and zachor were said simultaneously. In a lengthy piece, he promotes the idea that words embody meaning and when there is a quote of someone where words are added to the quote, or changed, this is not a contradiction. The quote is merely elaborating on what the speaker meant and was embodied by his words.

The ibn Ezra holds that Hashem only said one thing, as it is written word for word in Shemos- "Zachor". The implication is to remember which day is shabbos so that you may guard the commandments. When Moshe repeated the dibros in Devarim, he did not repeat word for word ("kadevarim"), rather he added or changed to explain the words more. One of the changes he made was to explain the endgame of zachor, which is shamor.

So when chazal say shamor and zachor were said as one word, they meant that the word zachor, which was said, embodies the idea of shamor, which was subsequently verbalized by Moshe.

Answer 2

Tehilim 62:12 says:

1. God spoke one thing, I heard two, for God has strength.

Rashi says:

...our Sages interpreted it as referring to [the maxim that] “Remember” and “Keep” were stated in a single utterance.

See also Rashi Shemot 20:8. In fact, Rashi (Shemot 20:1) says that all the 10 Commandments were originally said simultaneously, and later individually repeated:

all these words: [This] teaches [us] that the Holy One, blessed be He, said the Ten Commandments in one utterance, something that is impossible for a human being to say [in a similar way]. If so, why does the Torah say again, “I am [the Lord, your God (verse 2)]” and “You shall have no…” (verse 3)? Because He later explained each statement [of the Ten Commandments] individually. — [from Mechilta]

See here for a breakdown of the differences between the 10 Commandments in Shemot and Devarim.