"And if one curses his father and his mother" — the Mekhilta notices that this verse uses "and," connecting father and mother together. Taken literally, this might mean the death penalty applies only when someone curses both parents simultaneously. Cursing one parent alone would not be a capital offense.
Rabbi Yoshiyah resolved this by turning to (Leviticus 20:9): "His father or his mother he has cursed." That verse uses "or" — either parent individually. This means the offender is liable whether he cursed his father alone, his mother alone, or both together.
The Mekhilta is doing something sophisticated here. It plays two verses against each other to construct the complete law. The Exodus verse, with its "and," might suggest both parents are needed. The Leviticus verse, with its "or," clarifies that either one suffices. The broader reading wins: "in any event" — regardless of which parent or parents were cursed.
This interpretive technique appears throughout rabbinic literature. When one verse uses "and" (seeming to require both conditions) and a parallel verse uses "or" (requiring only one), the rabbis generally followed the more inclusive reading. The Torah was understood as a unified document where parallel passages illuminate each other. A restrictive reading in one place could be opened up by a more expansive formulation elsewhere. The result was a legal system built on synthesis, not contradiction.